I can't say that PSG will be able to win the Champions League next season if they maintain their performance because other clubs might have increased the strength of their teams with new signings. PSG haven't signed any player yet, because they feel it's not important which might later become a problem for them next season. This was how Leverkusen thought that they are the best and don't need any additional players when the won Bundesliga title.
Playing in the Bundesliga and playing in the champions league are two different competitions and every team approaches each competition they’re participating in differently. Just because Bayern Leverkusen won the Bundesliga title and got qualified for the champions league, that doesn’t mean those same players will be able to win the champions league competition with the same strategy, they are two different competitions with the champions league more fierce than the Bundesliga, so they won’t get to win it easier if they’d won the Bundesliga so easily for them.
As for PSG, if they were able to win the champions league trophy with this same players they have currently in the squad, they can just put more efforts in the champions league next season and dominate it once again if they manage the players well once again. The champions league is the highest flight competition in Europe. So if they’d won it the last, they can still win it again with good approach. Real Madrid have won it back to back before, so I don’t see why PSG can’t win it also, even without signing new players to the club.
Of course there is a huge difference between the bundesliga and Champions League, however it all mean that Bayern Leverkusen upon qualifying for the Champions League after winning their domestic League, can still win the Champions League with the same players and the same strategy, haven't watched the Champions League for several years, the winners in most cases are not easily predicted, so anybody can win. Even the PSG that won the Champions League, how many of us knew that they are going to win the Champions League, is the same PSG that has been trying for several years to win the Champions League but just was lucky enough to win it this time.