United were competing at the top table of European football with Wes Brown and Darren Fletcher in the first 11; it's since English teams STOPPED utilising as many British players that they've stopped getting anywhere near the latter stages in Europe. Chelsea had 4 English starters in their 2012 final team, and that was without the suspended John Terry as well.
The key element of success in football is IDENTITY; tactics, styles, and strategy can vary, but the common element in all great teams, those who perennially win, is identity and that's only generated by a certain number of domestic players.
Look at Real Madrid under Zizou; he's integrated Nacho, Carvajal > Danilo, Isco, Asensio and Lucas feature prominently and generally ahead of James....he's neutralising the perpetual golactico, mercenary mentality and creating an authentic Iberian identity that has seen them look like the most balanced, complete Real Madrid team since the days of Hierro, Raul and Morientes at the turn of the century.
Gary Neville wasn't Maldini, but he gave United more than Maldini would have; he loved the shirt, fought for the fans and for Manchester (plus England when playing in the CL) Ferguson generally avoided mercenaries and instead opted for players who owed their opportunity at the top level to him and United; those players will fight for you.
Yaya Toure, Silva and Aguero are arguably better than anything Fergie ever signed, but in the 7 years since they've arrived, City have won one league title by a clear points margin, and even that came in the post-SAF vacuum where Liverpool almost won it, and a mediocre Chelsea and even Leicester managed to pick one up.
One of them has handed in various transfer requests and booted off over a late birthday cake, and the other two haven't bothered learning the language; they have no real passion for the club or the City, and that sums up why they've failed despite having the best collection of (mercenary) players in the league for the past 7 years.
Kane isn't good enough? Ok, you're a lost cause. Go and masturbate watching Dortmund players on YouTube.