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.
Nothing is easy in football PSG struggled for years before finally lifting the champions League trophy last season if they want to defend it they have to be ready to outshine the opponents that they defeated last season though it's not going to be easy one for them because teams like Real Madrid and the rest of the strong teams will be coming back stronger for the competition but if PSG is lucky enough to meet teams they can easily defeat it they cross the quarter finals they can win it back again if they put up a good performance.