Yes, Chelsea have really improved and this year they can return to the ranks of the big clubs in the Premier League and in Europe. They have amply demonstrated this and are still demonstrating it. I, on the other hand, think that Paris will suffer a regression, citing the team they beat.
PSG, who were defeated by Chelsea in the Club World Cup, will not immediately experience a setback when competing in the Champions League next season. PSG will also strive to win the Champions League trophy again, although it will be very difficult for PSG, as several other teams will also be doing the same next season. But generally speaking, PSG will not be immediately weakened just because of their defeat to Chelsea, although I also admit that Chelsea have indeed made quite positive changes recently.
Each competition has a different atmosphere and motivation, which is what makes each competition difficult to predict. Chelsea's ability to beat PSG in the CWC final doesn't guarantee they'll beat them again in the Champions League. I vividly remember last season when Liverpool were on fire, but lost to PSG, followed by Arsenal and Aston Villa.
It's undeniable that Chelsea is currently improving. However, next season's competition will be tough because every team has made improvements to become even better. PSG will be the same, as the defending champions, but there's no guarantee they'll advance further in the Champions League. They could be eliminated in the league round or even fail to reach the round of 16. Anything can happen in football, especially with the new season yet to start, making it very difficult to predict. But what's clear is that next season's Champions League will be even more exciting, and this will be a new format for the second season.