The Champions League is a very different tournament. Only Real Madrid has been able to win it back-to-back. You can't use the form that PSG are in the CWC to determine the form they will be in the Champions League next season. A lot of teams will strengthen their squad in this transfer window, and they will become better than they were last season.
The performance of PSG at the Club world Cup cannot be used as a yardstick to tell how they will perform in the Champions League next season, though there is also prestige and aura in the Club world Cup but the vibes of the Champions League has its own unique feelings. Apart from Atletico Madrid, I don't think PSG has really faced a very competitive team in the CWC because despite Botafogo winning them but it doesn't count to say that PSG met a strong opponent. The Champions League season will be tough next season but we should not doubt a possibility of PSG winning the Champions League back-to-back because they have a strong team with good team spirit. Clubs will surely recruit new players ahead of next season's Champions League bt sometimes the smartest doesn't become the favorite. However, the CWC has its own energy and I think PSG is using the CWC games to warm up against the UCL next season.