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.
Defending the champions league is usually not an easy one for any team, even MADRID and Manchester City who are seen as the most holders of the title did not have it easy when they had to defend the title, and PSG as a first timer, they too will find it tough because these other big names in the league will be coming for it this season with a reinforced capacity to make sure they win it back ot at least get to the finals of the league. Most of the PSG plans and strategies will be aiming at the champions league this season.