Actually, their inconsistency began last season already and then Messi and Neymar now left and I would say that their replacements are inferior. Dembele can never replace Messi and Kolo Muani can't replace Neymar although the latter has been injured a lot, so this might not be too much of a difference compared to now. But the general inconsistency that PSG had in their game started last season. But especially Dembele is a player who has inconsistency in his DNA. He can never perform on the same high level for several games in a row. It is just a question of time until he will have the next crash in his performance.
The first half of last season is very good for PSG but things changed to be worse and terrible after World Cup 2022. After PSG were knocked out in Champions League, they started to be uncontrollable and their fans treated some star players very bad. Messi and Neymar were most favorite targets of PSG fans when they needed to pour their angry on some players.
Messi and Neymar did not happy in the past season and they together left PSG in the summer transfer. PSG replaced those players with Kolo Muani, Dembele and Asensio but not all of those players satisfy the club and fan expectation so far. Among those players, I only see PSG made acceptable decision to buy Dembele and Asensio but I disagree with their choice to buy Kolo Muani.
That French player wasted a great chance in last minutes to help France winning World Cup 2022 in a final match with Argentina in a face to face situation with Argentina goalkeeper. PSG even spent very expensive cost for Kolo Muani and I think it is very unwise transfer decision from PSG.
That is right the first half of the Ligue 1 season last year was quite ok. They had 15 wins, 2 draws and 2 losses. We could debate whether they should lose any game at all with their squad, but ok. It is still a good performance.
But you said something that is very important. When they got eliminated from the Champions League, it was as if they were lost. That was proof that the only thing PSG is playing for and that also applies to the players, is the Champions League title. They don't really care about the Ligue 1 title because that is almost a given. But once they get eliminated from the Champions League, the season can already be considered a waste.