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.
While it's hard to believe they'll win the French league with substitute players, it's important to remember that they're the best team in Europe. They won't be using substitutes, but key players in matches. The French league isn't to be underestimated; it's not a simple league. It's a very difficult league, and every team has a chance to win the championship. Paris won't do anything to diminish their chances. I'm sure they'll field the best squad in every tournament.
PSG is indeed one of the strongest teams in Europe, but not the best team. It's not because they won the Champions League that they are considered the best team in Europe.
PSG can also be said to have had a good moment when their strong competitors experienced a drop in performance last season. You can see what happened to Madrid and Man City last season, which became an advantage for PSG. Ligue 1 is always simple, with PSG dominating.