Haven't seen many young players that Allegri over looked who then went on to be stars elsewhere. His two big clubs were largely mature squads and the few somewhat talented young players in them played and did well under him. Kulusevski if he continues to do well may be the only one I can think of. But Pogba, Balo, Shaarawy, Dybala all featured regularly.
The preference of older players despite having young very talented players applies more to Mourinho who overlooked De Bruyne, Salah and Lukaku at the same time. Allegri is far from that.
Capello clearly underperformed in Europe given the quality of the squad. Yes most of the players were in decline and didn't thrive after they left but we all saw Ancelotti win the CL a year later with a squad just as old. And people like Ibra, Trez, Ale, Buffon and Cannavaro clearly had some top level football left in them. Ibra went on to become a legendary striker and Ale +Trez were top scorers in the league a couple of seasons or so afterwards. Buffon kept on giving and so did Cannavaro.
Capello himself never coached at the top level since that time too you'd have seen him bounce back if he still had it in him. Legendary coach in his days but we got him at the end
Italy has structural issues. Poor stadiums, training grounds and low funding to all football services. English players have improved because of the good funding and foundations to football and that's why they play regularly in the EPL despite the tougher competition for spots and the fact that they compete for spots against players from across the world and their clubs being able to buy anyone they want. The infrastructure is strong and produces strong players. Italys infrastructure has been broken for years and so naturally results in less talents.
I'm not talking against you but I want to state my view with capello's juve and also restate again what I've been saying in the past week or so about the repeated failures of this team in europe :
that capello underperformed in europe in almost the exact copy way conte and allegri ultimately underperformed in europe with juve, remember that juve had 5 1-1 draw streaks in cl group stage under capello? against average european minnows? isn't that so similar with some european adventures that happened under conte and allegri? let me clarify the allegri case first, yes he did manage to get to the finals, that's the high points, but you can't ignore his low points either... now what all those low points from different coaches and different era and players have in common? that the team played somewhat similarly... slow static boring football that either can't break down smaller teams or getting over run by better teams... you can even extend that to how the team performed under sarri and pirlo too, so it's not just the coaches fault but also the management fault, we keep signing players that are unsuited to play better football and keep neglecting players that are required to play better football... worsen with the fact that italian coaching mentality that's allergic to young players also factors into this.
the most notable thing about the type of players is that this club somehow has an aversion to attack minded dynamic wing players, before chiesa and cuadrado we have no notable wing players since the days of krasic, and then over and over again we somehow let players like coman, kulusevski, spinazzola and cancelo go just because they are deemed not "balanced enough", also in case of players like kulu and coman they were deemed not good enough all while they were often thrown into the field without systematic support from the rest of the team, putting a winger with slow static midfielders and backpass master of fullbacks that aren't there to compliment the wing play... I also see that this one is on allegri too, because that's what he did and still does...
if today we have those 4 players on this squad, I believe this current team would've played much better and achieved better results... but no, somehow over the years this team and allegri plays and at many times prefers to play with mandzukic as LM, or matuidi behind cr7 or danilo behind kulu or rabiot as LM etc... this team also keep acquiring CM and somewhat obsessed with SS over wing players, we persistently keep acquiring the bentancur, the matuidi, the emre can, the lemina, the rabiot of this world while keep neglecting wing players....
we see over the years that players like dybala presents a tactical challenge to integrate and complement the more modern play style but now apparently we're obsessed with raspadori, another player who is just like dybala and not worth it, also that we keep obsessing with players like pogba and sms while the sum of money needed to bring them would be better invested into 2 or more modern dynamic wing players...
messi, cr7, di maria, robbery duo, hazard, mbappe, griezmann, salah, mane, sane etc, even the 2nd rate wingers like dembele, son, sterling, carrasco, felipe anderson etc, juventus is allergic to these kind of players... we can't buy them at high prices? sure... but we also don't even try to find them at the bottom of the pile!
similar situations happens over and over again with the fullbacks too, we keep persisting on backpass masters like asamoah, sandro, danilo etc... the one time we got dani alves he was 34 years old and still took part in getting the team into another cl final, then only for in the next time we thrown away cancelo for danilo... and then people wonder in the past few years why our attacks always come from cuadrado alone, lol... aside from cuadrado, there has been what can be equated to zero efforts at acquiring another dani alves and the likes, on both sides of the field.
both the coaches and the management behind transfers are at fault for this... allegri for not even trying to create the team that can support those kind of players and the management for keep neglecting those kind of players...