Nobody said that Barcelona was a weak team overall. It's just that they've been through the early stages of the season, and it's not just the crisis they're going through I think Koeman has had a hand in this responsibility. Since Xavi took over he has been able to make the team better. That means something is wrong with the way Koeman manages this team.
There are many factors that Koeman considered a failure in managing Barcelona. Especially in terms of money management, Koeman spends too much on less productive players. And when replaced by Xavi we see money savings by taking advantage of the portion of players who are quite qualified. Therefore, Koeman is considered wasteful and does not prioritize the quality of the players. Now in Xavi hands, we see a lot of positive changes that could have made a difference when Barcelona was under Koeman.
I didn't know Koeman made the transfers, I assumed that the president and the GM etc etc made those transfers and just told koeman to play with whatever they gave him, sure there were suggestions who to get, but not directly made the decisions.
If it was like you said, like Koeman was the reason they paid that much for the players, then I have to say it is definitely a wrong move for sure and makes sense for him to lose his job not just over bad results but financial situation. Xavi didn't really improved the financial situation all that much yet, he needs time for that, they need to play with very cheap players to keep it going in order to pay it all.