Let me put it this way.
A new manager + board duo will impose a new order of things. From the top. Non-negotiable saint rules. Certain standards of fitness, training intensity, in game tactics, pressing, work-rate, closing down opponents, blocking passing channels, doing runs off the ball. The standards City, Liverpool, Bayern currently operate at. Or even lower level teams like Leipzig, Dortmund.
These things are tough to do on a week to week basis. We won't actually fire veterans per se. They will retire out of being unable to cope with the requirements of playing for a club aspiring to be the best in terms of fitness, athleticism, professionalism. They will exclude themselves, and will fall behind like the past it and the old usually do.
But if the veterans acquire the possibility and the means to influence these things, and slowly drop the standards they have to comply and thrive to get to weekly, then they could hang around for more, eating at the club and earning tens of millions. Basically becoming some sort of parasites. Until they get schooled, like the slackers and unprepared do, because the standards you go for are much lower than the standards the competitors have to reach. These things show. They always do somehow. They showed under Valverde too, Roma, Liverpool back to back humiliations are proof.
This is what happened to this club. Playing for this club in terms of professionalism, fitness, athleticism, tactical compliance with the instructions, has gotten waaaaaay to easy.