I don't really understand whether every club's failure in prestigious competitions is the coach's fault. Then what about a club like Paris Saint Germain who have never win the Champions League, for example, even after change coach?
Excess star players is also not always good for a team, it will actually create new problems with the team
The coach is the first person to be blamed when a team is not playing well, and it because it is his tactics and methods that the players go into the field with, the club owners cannot sack all the players in the club, it is not possible, but they can sack the coach who is only one individual. Though a club's failures is not only the coaches fault, but they are the ones that get sacked nonetheless. Psg's problem may be because of they have too many star players who are not following the coaches instructions and playing only for themselves, but now they have failed again to win the champions league, they may sack Galtier and only a few players will leave.