Anyone that feels PSG can defend the title next season is not use to watching Champions League. If not they will understand that all clubs in this competitions are champions and you shouldn't underestimate any of them. PSG was underestimated previous seasons but look at how the wonderfully won the title this season. They can't win it again next season. This is why the league is unpredictable and interesting.
What you said makes sense because the Champions League is a tough league and even more difficult than any domestic league so there are always surprises in the Champions League every season. In addition, every team that tries to become champions in the Champions League or wants to defend the Champions League must have above average quality players along with a coach who is more experienced in planning his tactics too. Because if a team only relies on good main players, it will also be quite difficult for a team to become champions in the Champions League because disasters such as injuries in the middle of the season are always unavoidable and that was experienced by Real Madrid last season.