The midtable teams in la liga is inarguably FAR ahead of Ligue 1 for the reasons DonAK has listed and a few more (like how lower table teams seem to routinely knock RM out of CDR, even if RM aren't trying as hard that still shows strength in those lower teams). And I'm completely discounting PSG/Barca/Madrid/Atletico/Monaco here, just talking about teams like Sevilla/Valencia/Athletic/Villarreal who have had success in Europe far beyond the 'equivalent' French teams.
On the other hand, sure, you can't really separate the bottom 10 in any of the top leagues from each other. One relatively crude method I can think of is comparing bottom 10 to top 10 across leagues. It is an inarguable given that La Liga and EPL are superior to Ligue 1. So let's start with that assumption and look at the point distribution of the league tables from 17/18.
The accumulated points by the bottom 10 in La Liga ranged from 20-50, which is more or less the same as Ligue 1's 26-51 range. EPL's is a much tighter distribution than both, ranging from 31-44. What this shows is that the difference in quality between the top 10 and bottom 10 in La Liga and Ligue 1 is relatively equal, while EPL's bottom 10 is closer to the top 10 than both leagues.
Given the (pretty much factual) assumption that La Liga is better than Ligue 1 and the difference between the top 10 and bottom 10 is roughly the same, that shows that La Liga's bottom 10 is correspondingly better than Ligue 1's bottom 10 based on how many points they were able to accumulate against the top 10's of the respective leagues. Whereas EPL's bottom 10 is better than both relative to their top 10, and EPL is clearly better than Ligue 1. So that also shows EPL's bottom 10 is also superior to Ligue 1. La Liga is slightly better than EPL but since EPL's bottom 10 is significantly closer to the top, there is a good argument to be made that EPL's bottom 10 is better than La Liga's.
That's a very rough, quick way of doing it. You could make way more detailed calculations, but it's one way of gauging the bad teams in each league.
Even Jair Ventura's criteria can be applied to show this. The bottom teams in France's Ligue 1 are extremely tiny clubs with no resources at all and tiny stadiums/shitty academies. Who finishes at the bottom in La Liga? Teams like Malaga, Rayo Vallecano, Granada, etc. who at various points in the past decade have been midtable sides. Athletic was a bottom tier side last year despite having multiple seasons in the top 5-4 in seasons past. Valencia, Sociedad, etc. have all had seasons in the lower end of the table sometimes excessively so.
Whereas in Ligue 1 the top 10 always tends to have the teams with at least acceptable resources (Lyon, Nice, Marseille, Bordeux, Rennes, etc.). The bottom 10 is always the minnow clubs like Caen, Metz, Amiens, Guingamp, etc. There is very little change in the makeup of the league table every year for the past 5+ years, unlike La Liga. Because the lower table clubs are extremely tiny with very little room to overtake those clubs with more resources. They are inherently extremely weak every single year without fail unlike La Liga's bottom 10 which routinely includes clubs that have rich history and consistently above average resources. Small clubs by nature vs small clubs by circumstance/performance.
I have read your post 3 times and I am still confused.
What number of points should bottom teams in France are supposed to win then, to prove that they are good, way higher than in La liga?
But then, points are in majority of cases spread in same patterns in all leagues with 20 teams.
For 100 seasons in a row, regardless if you have a one horse race, 2 horse race or 5 horse race.
Regardless if a league was very strong or weak in that season.
Ok, there are slight differences in points under some circumstances, but in general, numbers are quite similar in all countries.
Do you remember that saying from England: if you have more than 42 points, you are safe, something like that?
So, it is irrelevant how strong the league are. With 42 points, you will be safe in 90 or 95% of seasons.
Note: I think that La Liga is still better.
How much?
Hard to tell.
Are our teams placed 11-20 better than French teams? I have no idea.
On the other hand, you could add a different test, the same as in EPL: if there is a weaker difference between teams, that means that a league is more even, right?
Like in stats, it is often better to remove extremes which are ruining the test.
So, let's delete top5 teams and bottom 3 teams who are horrible.
Now, check teams from the 6th to 17th place in France and Spain over the last 5 years:
France:
2018:
6th: 55
17th: 38
= difference +17
2017:
6th: 59
17th: 37
= +22
2016:
6th: 58
17th: 40
= +18
2015:
6th: 63
17th: 42
= +21
2014:
6th: 60
17th: 42
= +18
Spain:
2018:
6th: 60
17th: 43
= difference +17 points
2017:
6th: 64
17th: 35
= difference= +29
2016:
6th: 60
17th: 39
= +21
2015:
6th: 60
17th: 35
= +25
2014:
6th: 59
17th: 40
= +19
France, total: 17, 22, 18, 21, 18=96/5=19,2
Spain, total: 17, 29, 21, 25, 19=111/5=22,1
As always with stats, you can draw several positive and negative conclusions.
If you want to claim that Spain is way stronger, these stats could say:
1. the difference between 6th and 17th team in Spain is larger because top 6 teams in Spain are too good, and bottom table teams can't get any points.
While in France, there is only one big team, so bottom table teams have only 1 team against whom they will surely lose, so bottom table teams are winning more points.
The same argument could also be in favor of France:
= this proves how teams in France are way more equal in strength and there are no easy games where you can win 8:0.
All teams are good and there are no easy games.
A guy from France could claim: bottom table teams in Spain are not winning too many points NOT only because Barca and Real are too good, but because those bottom table teams are of a real poor quality.
You could add goals conceded in maths and say:
Bottom table Spanish teams are conceding more, but it can also work both ways:
Spanish guy could say: bottom table are conceding a lot, because Barca, Real and top teams are TOO GOOD and bottom table teams need to concede a lot of goals.
French guy could say: bottom table teams in Spain are conceding more goals because they suck and are of a weaker quality than French teams.
Again, I DO think that La Liga is better, but I don't think that French league is THAT bad so that we should call it a farmer's league.
Or that an assist in France doesn't mean anything.
Also, no offense, I still don't get your idea and what were French bottom teams supposed to do to prove in your example that they are better than Spanish teams, plus, if that is mathematically possible since patterns of points distribution is quite similar in any league with 20 teams :/
And as always, not to mention that stats can often be used in 2 ways.
Same numbers could back up lots of opposite theories.
La Liga on the other hand you have this sort of thing consistently. Teams like Eibar with literally no resources pushing their way to the top 10. Rubbish teams like Girona, Levante, Getafe, etc. ending up higher than Valencia or Real Sociedad or Athletic Bilbao. It happens every year. The bottom clubs are actually capable of competing above their means.
This will sound that I am arguing just for the sake of arguing, but imo, AGAIN, such numbers can back up 10s of opposite theories.
You say that Eibar with no resources playing well=proves how strong La liga is.
I could reply: No. That actually shows how crappy midtable La liga teams are if a club with no resources can outplay them easily.
And how that can never happen in France.
One more time, I DO think that La liga is better.
But these theories based on number of points and anecdotal stories about a single club...
I have checked Uefa's coefficients from the past now:
https://kassiesa.home.xs4all.nl/bert/uefa/data/index.html
My point is that these rankings are HEAVILY influenced by results of 2-3 top clubs, especially in recent 1-2-3 years.
Now, 2005:
https://kassiesa.home.xs4all.nl/bert/uefa/data/method3/crank2005.html
1. Spain
2. England
3. Italy
4. France
5. Germany
So, in this moment, Getafe was better than English bottom table teams.
And France was better than Germany, for example.
Then in 2008, England jumped on a place no1:
https://kassiesa.home.xs4all.nl/bert/uefa/data/method3/crank2008.html
So, Getafe is now in theory weaker than English bottom teams.
But, France still better than Germany.
And Italy on a number 3 even though they had Calciopolli and dropped a lot in their quality.
But, these coefficients are slow...
2010: Germany jumping to No4 infront of France.
2011: Germany jumping to no3 suddenly infront of Italy who finally lost points.
But has German league actually improved? Not that much, it all depended on Bayern's results.
2012: England still on top, even though we have Messi and CR7. France dropped from no4 to no6 in 4 years. Portugal is now 5th.
2013: Spain finally jumping to a place no1. France still 6th.
2016: Spain is the 1st, but Germany jumped to a place no2 and England dropped to a no3.
Now, a question: does anyone sane thinks that German league in total is better than EPL?
Or that top, midtable or bottomtable teams in Germany are better than EPL teams? No, of course.
Germany is all about Bayern. And some CL results from BVB, while EPL teams struggled in a CL and that's it.
But a quality of Bundesliga teams didn't raise suddenly.
Also, in 2008: Germany was 5th, behind Italy and France and in 2016 they were the 2nd.
Has anything changed in a quality of German teams outside of Bayern? No. The only difference: Bayern sucked in late 00's and thus Germany had poor coefficients. Now, when Bayern started to reach semis of a CL and won a title in 2013', Germany jumped to a 2nd place.
2017:
https://kassiesa.home.xs4all.nl/bert/uefa/data/method4/crank2017.html
My point: you need 10s of different factors to estimate strength of a certain league.
Uefa's rankings are based on a few top teams in a CL mostly.
Number of points in league, from my example, can be explained in 10s of different theories.
I still think that we know very little about a strength of a bottom 10 teams in Epl, La Liga, Bundesliga, Seria A and Ligue 1.
We can estimate a strength of top teams to some extent.
But what is happening in a bottom of a table, more or less, nobody knows.