Yes, it will not be easy for PSG to defend their UEFA Champions League trophy next season because a lot of big clubs are serious in the transfer window, strengthening their teams, and it wouldn't be easy for PSG to beat some clubs in the next season's UEFA Champions League. For now PSG has no rival in the French league, and if they play with their second eleven, they will still end up winning the French Ligue 1 title because winning the French Ligue 1 is like their birthright.
The challenge in the Champions League is indeed much more difficult, as there are many teams that are likely much better, and to retain the trophy, Luis Enrique needs to do a lot, so it won't be easy. PSG's motivation will be high to defend the title, but considering the strength of the players currently available to other teams, it won't be easy.
Speaking of Ligue 1 there will probably never be a major challenge for them, as the strength of other teams in the league is almost unmatched by PSG, so defending the Ligue 1 title will be much easier than defending the Champions League.