Koeman is a good coach and you should not blame all the problems of Barça on him. Yes, during the time when Koeman coached the team, the club did not show good results.
Barca's problems began when Bartomeu began to steer the club, that's who ruined the great club.
At the moment, I generally have big doubts about Koeman's fault - Xavi came and from the point of view of the result, nothing has changed at all, Barcelona played like an average player and continues to play the same way. Also, I think that Xavi's idea of returning to the old style of play is harmful and will not bring any profit even if he can establish this style.