Nothing is impossible in football, and I believe that If PSG are really desperate to defend the title, they can win it back-to-back. Personally, I don't think that's possible for them to achieve next season, because the king of UCL will be back in form and the trophy will be their most focused trophy next season. Real Madrid has been winning UCL in and out since the past four season.
The UEFA Champions League competition isn't French League 1, which PSG dominate every season, and even though PSG are desperate to defend their UEFA Champions League trophy next season, it's not a guarantee they will defend it. The UEFA Champions League is a difficult competition, and if PSG want to really defend their UEFA Champions League trophy, they should put in a good performance and be consistent in winning their matches in next season's UEFA Champions League because a lot of clubs have improved their teams, which will make it more difficult for PSG to defend their UEFA Champions League trophy.