Yea, because Spain and Barca have looked awesome the past half decade with Busquets as that undisputed starter right?
Busquets started getting exposed at Barca the same as he did in Spain- when Xavi and Iniesta were older and no longer at their best. No one making up for his multitude of faults anymore.
Managers are people too, and keeping Busquets as our DM has been a mistake they've all made, and the similar lack of success they all shared is very apparent.
It is not that easy. Busi was never really part of a well functioning team since 2016 or so. He is part victim, part perpetrator.
There is no doubt for example that Modric would be badly exposed in a different team. But Madrid did a very good job at hiding his limitations with proper scouting, having a world class manager, the best striker in the world AND the best keeper in the world. Man is basically sandwiched between world class quality on both ends. Barca didn't do that with some veterans, and had abysmal transfers. And the biggest blame for this is not with the players themselves, but with the previous board.
Have you listened to some top class players talk? They think they are as good as ever even at 33-34. Change doesn't come from within them, but is forced upon them by competent and forward-thinking management. I bet Casillas felt betrayed and persecuted by Florentino, even though he was for sure done.
Players and legends don't step down from a high-money, high-status positon out of nobility and sacrifice. They are forced to do so by top class management. When you have a clown in charge, they basically take over the club. Florentino did the same mistakes in the Galactico era, but learned from them and became a truly elite president who put the club first ahead of anyone.
I have no idea frankly how well or how bad would Busi have worked under a proper team, because I haven't seen us having a proper team in about 5-6 years.