City just have so much strength in depth, it doesnt matter if they miss key players. KDB for example, much lauded as the bestest player in the premier league bar none, missed a chunk of the 18/19 season and last year yet they were able to win the league on both occasions. Take the best player out of liverpool (eg VVD) or chelsea or any other PL team and watch them struggle.
They also plan really well when it comes to replacing their stars. Yaya toure, kompany, david silva, fernandinho have all been replaced with quality players. Even Aguero, one of the best strikers in the PL left city yet it seems they havent missed him one bit! Meanwhile United are still trying to find a capable midfielder, right back and center back! Pep has been a huge part of their success, but its safe to assume that even when he does leave, they will carry on winning titles and the structure in the football club is so good.