I think Lionel Messi was not used well by the PSG team. Because with Lionel Messi you can achieve something unique. I think the PSG coach doesn't know how to use Lionel Messi. If Lionel Messi got a coach like Pep Guardiola, he would have created history by now.
It may be a good point that Messi was not well used by PSG club coach. Messi's coach needs a person who understands football better than Messi and a coach who takes into consideration how to organize the team according to the opponent. In this case, PSG club needed Pep Guardiola or Zinedine Zidane could have been a good choice for PSG. But PSG failed to do so, resulting in a conflict between their club's players that never ended.
If Zinedine Zidane were to take charge of PSG I think he would have been able to remove the chaos in the dressing room and if the chaos in the dressing room was gone then PSG would definitely be better on the field.
What do you expect getting the manager from nice or lille or whatever. This dude wasn't all that great even when he was hired and of course he utilized Messi terribly as well. If you want to win UCL then you need a manager that has won UCL before or at least got close to it, Galtier was a horrible move even when I first heard about him, and it is still a terrible manager for PSG today. He is a great player for some midtable or even a top table French team but not PSG, they have other ambitions.
I think they seriously up the price on Zidane, if he wants world record salary then just pay him. In any case, Messi is gone now so there is nothing they can do about it, but it was obvious from the start of the season that they will be bad this season, they got lucky with league title.