That's in reality the biggest problem that Ligue 1 has. Not winning games has no consequences for PSG in 9 out of 10 seasons because there is no other team that would be able to achieve more than 80 points in a season. It is not that it never happens because it does happen as we can see in the last few years. But those exceptions happened when, for instance, Mbappe still played for Monaco. But then PSG can still play on another level in Ligue 1 when necessary to win the title. It is still not serious competition after all. Serious competition would be when PSG can't win the title at least two years in a row. I don't think we are going to see that in the foreseeable future.
PSG domination in Ligue 1 is even bigger than Bayern Munich domination in Bundesliga. These clubs have same style to buy best players from their competitors in domestic clubs and this strategy helps them to destroy competitiveness of direct competitors and help them to improve their squad strength.
In Ligue 1, what you said is true, other clubs are too weak and barely can have good seasons to challenge PSG and beat this giant club. PSG many times had their bad matches but they still completed those seasons with titles because other clubs are too weak to take advantage of PSG's poor performances. In domestic league, PSG easily won titles but it becomes their weaknesses when they play in Champions League. Being familiar with lower quality of matches in Ligue 1 and weak opponents there, they don't have enough quality, carefulness and preparations for tough matches in Champions League.