Going to the top from the bottom is an easy job, staying at the top and trying to maintain position over there is the hardest. Me, as a person who has watched lots of historical champion's league editions, I won't be surprised if PSG got eliminated from the UCL in the earlier rounds next season.
I don't think what you're saying is very realistic. If they don't face one of the tournament favorites, they will go as high as they can. Because we saw in the World Cup that Paris Saint-Germain is competing very well. Everyone who expected the team to decline after losing Mbappé last year looks at this year with surprise and admiration. Expectations are high for them this year. Since they were Champions League champions last year, their fans want this success to be repeated this year.
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.
I agree with what Bitcoin_Arena said. I would not be surprised if PSG get knocked out in the quarter-final of the Champions League, and it won't be the first time. Real Madrid, which is always the favourite to win the Champions couldn't go past the quarter-final after winning the tournament and adding Mbappe to their team, even though everybody tipped them to be the best team in Europe after the inclusion of Mbappe. The Champions League is a different tournament, very difficult to win and even more difficult to retain.