I don't know what's worse. Messi stays and Bartomeu leaves or Messi leaves and Bartomeu stays. It seems to me that it would be best for the club for both of them to leave.
Bartomeu is already scheduled to leave. Why would anyone do that to Koeman? If he came, he for sure had a clear agreement with Bartomeu who guaranteed him full powers to set the foundation for a rebuild of the team. Bartomeu is scheduled to leave in about half a year. Him remaining president ensures Koeman the continuity he needs to continue the work he has started.
This is pretty clear to me. Bartomeu started a rebuilding process after the 8-2 historical embarrassment (something he should have done years ago) by investing Koeman with full powers, something nobody since Lucho had. Koeman is a totally different animal in terms of mentality, ambition, authority, than guys like Valverde and Setien. This irked the veterans the wrong way, because they felt signaled out. Especially once Koeman started to make some things clear that the way of doing things in the team will change.