It's not a problem if you dominate Ligue 1 even for a long time without tough competition from other teams, such as Bayern who control the Bundesliga, but the problem is that they don't build this team strong enough to compete on the European stage like what has been done by Bayern Munich in the last few seasons.
But it is a problem. It's proven that if your league is tough, then you produce really good teams and players. Competition makes teams try really really hard. Bayern at least has consistent challenge from Dortmund and even in cups fight very hard with others but PSG can clean sweep everything if they just try.
They have been having it too easy, so even with superstars they didn't try hard anymore. They lose or draw because they take things too much for granted.
PSG spent big, it was strong enough to win everything, but it wasn't "shaped" by good competition.
I agree with you that the tougher the competition in the domestic league, the more motivated a club will be to improve. I'll take the example of the Premier League for example, they get tough competition from all clubs, we can't easily predict matches there, because there clubs that are considered weak can become a stumbling block for big clubs. And we can see what they can do in European competitions as a result of the intense competition in their local competitions. Unlike Ligue 1, we can see what they can do in European competitions, especially the Champions League. They can't say much there.
We can't deny things like this, because after all this is part of the mental formation that will be very useful.