PSG will easily win Liga 1 again, playing happily. They don't even need new signings.
Even if they compete in Liga 1 with their current squad, and even with the reserve players, they'll win the title again.
PSG's main goal will be to win the Champions League again.
Things won't be easy this year. Real Madrid has restructured, and Barcelona is already a very good team. City will be very different from last year. Liverpool is extremely prepared. I don't expect PSG to win the Champions League again this year.
I am also sure that PSG will not have difficulty in Ligue 1. But to say they play with reserve players to win the title, I think it's an exaggeration. If it were very easy for PSG to do that, they might do it and prepare their main squad to focus on the Champions League. They don't do it to not give other clubs a chance. PSG is strong in Ligue 1, but that doesn't mean they are unbeatable. There is no doubt that they can win Ligue 1. They have the advantage of rotating players with a tight Champions League competition schedule.