It's irrelevant if it's Setien or somebody else. The veterans run the team and the manager has to listen to Messi, Suarez, Pique & the other big shots in the locker-room.
Xavi needs to stay away until the board gives him absolute powers to make every technical decision in the team. If he comes too soon and is not cut-throat like Pep early on, they will try to 'manage' him as well.
It's clear the current board doesn't want to upset the vets. In these conditions, no top manager would touch us. The ones that would are Valverde, Setien types. Guys from Bilbao, Betis, that never coached a tier 1 club (or even tier 2 for that matter) in their life, and for which Barca is too good to pass, in any conditions.
Not necessarily bad managers, but managers willing to accept compromise to keep their "prestigious" job. Even if they have much less power than a real top manager has at other clubs.
That is why we are where we are. Weak board, strong selfish veterans (who happen to great legends), and second rate managers. It's a cycle that is not breaking without a new board willing to take risks and attempt to rebuild this side.