PSG might not be able to win the Champions league but they'll do fine in the tournament maybe reach the Semi-finals or so, some people think they'll be weak cause of the defeat to Chelsea but I think they'll learn from it and move on, their squad is still very strong they didn't sell any players so if they can work as a team as they did last season then getting to the finals next season is still possible.
If you noticed they still beat some big teams some big teams in the Club World Cup before they got to the finals to meet Chelsea they were even in a tougher situation than Chelsea in that tournament and if they could scale through to the finals then they could do similar next season put their heads together and win it again though it would be tougher this time around.
The defeat to Chelsea shouldn't be taken as a complete benchmark, PSG will still be considered the favorites for next season Champions League. PSG performance hasn't declined significantly in fact, I believe they're still at their best, just unlucky in the Club World Cup. While it's true that back to back Champions League titles are extremely difficult, it's not impossible.