...
PSG at the moment have an enviable squad depth, all they need is to enhance it for next season due to how expectations on them would be very high as defending champions in the champions league. Imho, Gavi is a quality player but he doesn't fit into the PSG kind of football with how prone he is to injuries he can't be reliable to be consistent in a season. What PSG needs a midfielder that is void of such injury problems and Gavi isn't that player.
Remaining successful is more difficult than being successful once. It has many reasons. Among those reasons are that some players lose their drive, their ambition to give all they have once again. Another reason is that your success doesn't get unnoticed and opponents prepare for it to take you on differently next time. You could see that when Leicester City once won the EPL or Bayer Leverkusen won the Bundesliga. The season afterwards felt several times harder for them. There are plenty more reasons and mental aspects play a role too.
Now at PSG chances are higher that they can maintain their level because they really are a top team and it has not been as big of a surprise as when a team out of nowhere wins something. PSG has been good for a while and now made it possible to finally peak during the right moments and win the most important games.
Whether they can maintain it or not is an important question. I hope so because they made the CL much richer in particular. Ligue they will win no matter what, but how they will be doing in the CL is important for them.