He’s not the only player under-performing though. Sancho, Casemiro, Varane and Mount are all having a difficult time. Eriksen did decent last season but is on a single goal and assist so far, he still seems to be performing at a good level but it’s not translating into goals/ assists because no one is scoring. Both Ronaldo and Cavani fell off in their second seasons in the past. Rashford is always oscillating between hot and cold patches. Apart from Bruno, none of the players seem to be performing at a good enough level consistently. Others that do perform like Aaron Wan bissaka and Luke Shaw seem to be quite injury prone. This is a huge sample size of players that are under-performing and I don’t think it’s fair to attribute the failures of the club to the players anymore.
I think it was more a case of Casemiro wanting a new challenge and the club respecting his wishes. Both Modric and Kroos are significantly older than him and are still playing, I’m sure he could’ve continued for a few more years as well if he had wanted to.