I know you are extremely big on the idea that European competition is all that matters, but not everyone believes this. It's not that Italian clubs didn't care, it's that they cared more about their domestic league until Milan arrived.
Yes the European Cup, UEFA Cup and CWC were always important..but Italians have always been nationalists about their football. The Europa League might not be taken too seriously by anyone today, but the Italians phone it in far more than anyone else.
It's not that teams don't try in Europe, but you far overrate European competition. League play, at least in good leagues, means just as much. A club like Napoli never had the depth to compete on two fronts.
It's not that the Italian league was overwhelmingly better, if that was the case then they should have sleptwalked their way to more international silverware, but from the mid 80s until the mid 90s, it was the best league in the world by a good margin. Even if the best clubs in other leagues were good enough to keep them at bay in Europe..aren't you the one who says the EPL is clearly the best league in the world today even if Spain has 2 stronger sides? What made Serie A such a big deal is the entire first half of the table was great.
And as said before, most of the Internationals listed to downplay Maradona didn't join the team until after the 87 double.
I am not saying that Europe is all that matters. But it is the only reliable (if imperfect) metric of the relative strength of each and every league. To merely claim that the Italian league circa 1987 was absolutely epic, is not nearly enough for my money. I have little doubt that it was the best in the world (Germany, anyone?) at that point in time. But the question for me is, by how much?
This is extremely important when it comes to the GOAT debate. Diego's legend, after all, is largely build upon two main pillars:
a) He carried Argentina to the WC triumph of 1986.
b) He carried Napoli to two league titles, against epic competition.
To be certain, there is a heavy element of truth in both of the above statements. But myself, and some others, are of the opinion, that there is also an exaggerated element in all of the above. For example, in 1987, Juventus was on the wane, due to several injuries to their key players (Platini in particular decided to call it a day after that season) and Napoli was not as weak as many make it out to have been. In 1983/84, the pre-Maradona Napoli, finished 11th in the table. In 1984/85, Maradona's first season, they finished 8th (a marginal improvement) and clear evidence, that no really good side, can be a one-man team. Further additions were required in order for Napoli to finish 3rd in 1985/86. And, like I said earlier, Juve had to suffer several injuries in order for Napoli to win the whole thing. Next season, Juve plummeted, and it was Milan who took the slack (not all of the big name teams were great the whole time) In 1989, Internazionale, walked the title (not so competitive then. 11 points difference, with old 2-1-0)
As for Maradona being able to carry average teams on his shoulders against basically any opposition. Why couldn't he even remotely do that in the old (much easier) European Cup? I am very well aware of the pitfalls of this argument, I am basically giving the man only 2 chances of succeeding, but isn't that what everyone does with Messi?
My point here is, that a player, can only work with what is in front of him. Both Messi and Maradona have excelled beyond imagination, and it is nigh impossible to compare the two. If this question is ever to be answered, then a much more consistent and precise methodology must be devised, than all the tired old arguments outlined above.
As for your initial point on European competition. I do firmly believe, that there is no better way of evaluating both clubs and players, than European competition. Domestic leagues, are restricted in their scope, and they will always remain a riddle to the football fan (how would Barca fare in the EPL, what would Arsenal do in Serie A, how would Messi fare in either England or Italy)
International competition (national sides) is seriously compromised by time (one WC every four freakin' years, one Euro ever four as well) and number of games (one bad call and you are out)
So yes, European competition (the European Cup especially) is for me the most serious benchmark in the world football.
*And again. What you are saying about Serie A circa 1987, I am more or less saying about today's EPL. Being the best league, is not about owning all and sundry, it is about being stronger overall, even if marginally.