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.
If you look at the titles that Bayern Munich have won in the Bundesliga in succession, Bayern Munich's dominance is actually stronger because PSG, although they have often been champions of Ligue 1, have not been in a row for a decade like Bayern Munich because at least in this decade Lillie and Monaco have managed to become champions even though it was only for one season because then PSG managed to reclaim the Ligue 1 title.
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.
And agree with this, the weak competition in Ligue 1 is one of the reasons why PSG did not manage to perform well in many seasons when they competed in the Champions League because they did not get strong pressure in local matches in contrast to clubs in the English Premier League and Laliga, with the exception of Bayern Munich even tought is in a league that is not too tight, it is quite successful in the Champions League.
Likewise, this season PSG will probably only reach the quarter-finals and semifinals of the Champions League, which is quite a step in my opinion.