Sorry, I've missed this.
I've never said I would pay 200M for Firmino even if he'd be available to us (I'm aware we can't buy L'pool players), I don't even like him that much. I was just asking why people in general are considering players aged 27 as "too old" (and those at 30 as "semi-retired") when we've actually had more success with players of that age than 20-22 year old kids who many here always want to buy. I don't have time so maybe BBZ can help me out as he has written about this historical comparisions in the past. That said, I don't share his "hate" towards younger players. In fact I've liked most of the signings we've made over the past few years and I've even supported/defended Gomes and Paco when most people have given up on them. And they are proving now in other teams they were not a fit for us but they are far from bad players. But on the other hand I also like more experienced players like Raki and Paulinho who are/were often criticised unfairly. Will probably have to add Vidal to that list some day but he's actually loved right now by majority here.
I think we need to find the right balance. We probably won't win anything with our first team being all over 30. But we won't win anything with just inexperienced kids either. We need guys like Busi, Raki, Vidal, Pique to give us experience and winning mentality. We need youngsters like Dembele and Arthur, possibly one or more of De Ligt, De Jong, Aouar, Ndombele,... (and maybe Alena/Puig/another La Masia kid will be good enough to at least be a squad player for a few years). Alongside their football skills those youngsters would bring fresh blood and motivation (because none of them has won much if anything in their careers). But where I think we're currently lacking is with players at their prime around 24-27 who have football quality (not just technique but also intelligence, work rate,...) to help us now and also to lead our "next team" when veterans will move on, leadership abilities (not every player has it but we should find some players who could become vocal leaders in the dressing room - right now I think MATS is the best suited to be our next captain once Messi&Co. will leave) and more experience than younger players mentioned before.
I know some players nowadays mature sooner and can be leaders of the team even in their early twenties but we still don't have many players in that age 24-27 range. MATS, Umtiti (who seems to become injury prone which could affect his longterm future at the club) and if someone considers Semedo/Roberto as unquestionable longterm starters. I don't. They are what they are at this point of their careers and I don't see much room for improvement for either of them (which would be needed) but they are still good players and will probably be kept around for some time unless we can find a clear upgrade for RB position which could be tough. RB might be the most deficiary position in world football right now. Obviously we shouldn't stop looking for improvement at that position though.
That lack of worldclass (or close to that) players at their prime years was probably one of the reasons why we overpaid for Coutinho (along with the club wanting to get another Brazilian star after Neymar left). I don't question his talent but he hasn't find his position on the field with the team so far and I don't think it's fair to only blame Valverde for that. Hopefully Coutinho and Valverde (or whoever will be the next coach) can figure this out sooner or later but if not we'll have to make a decision with him.
I also think you are overrating sell-on value when it comes to Barça. We are historically not good at selling players regardless of their age. If we buy/produce players who establish them as starters we are usually keeping them until their midthirties and then they leave for free or retire like Iniesta, Alves, Xavi, Puyol,... and the same will probably happen with Pique, Busi and Messi. We don't sell starters/key players at their prime unless they are not a fit with us (Ibra, Cesc) - so we can't really sell them at their highest value - or they leave on their own (Neymar). The last (former) star who we actually sold for a solid fee (at that time) when he was already on his decline (but more due to his life style than just age) was Ronaldinho.
On the other hand, when players are not good enough to establish themselves as starters (or at least valuable squad players) we usually sell them below their market value (which is not that high anyway) or loan just to get rid off them. We can't sell 24 year old Paco for 80M like Madrid did with Morata. Yes, our high wages play a big part here but it can't be the only reason. We probably won't get more than 40M for Gomes and this will only happen because he'll "prove" himself in EPL this season. If we'd sell him last summer we'd probably get around 20M (so loaning him to the league with most money was actually a smart decision longterm; maybe we are learning something?) So would their sell-on value really change that much if Paco and Gomes would be 3-4 years older? I don't think so. Maybe for 5-10M but not drastically. Of course it's possible we wouldn't even buy them at 25-26 because we were betting on their longterm "potential" at 22 but there would be other names/players instead who could (and probably would) go down their road. We did a solid job by selling Mina for around 20M profit but this was also because of World Cup hype and 30M was probably closer to his market value than 12M we've bought him for (there were rumours Dortmund and some other clubs were willing to pay 25M before we got him) so we did better by buying him well below his market value in January than selling him a bit about the one after just 6 months. That said, I like that we sold him and Digne (and loan Gomes to recuperate his value) to Everton and we should continue to send our rejects to rich PL clubs instead of loaning them (with buy options that are never used) to Serie A or smaller La Liga teams.