I think PSG's defense is not that good, but even so, it is very difficult for other teams to compete with PSG. PSG is far above the other teams in the league both in terms of quality and financial power.
PSG started the season badly but over time they started to play better. I criticized PSG a lot at the beginning of the league but now they are playing better football. So I won't criticize them too much.
Nice currently have 35 points and they are 5 points behind PSG. They cannot close this points difference easily. As the league progresses, PSG is less likely to lose points. The players and the team are adapting more and more to each other and they are winning series. I can say that Nice will have a difficult task to follow PSG.
That's one important point about the game of PSG. It is not that important how many goals they concede since they will most of the time score around 2.5 goals per game. It doesn't sound all that much, but the difference is that a team like Nice scores 1 goal on average and if they concede 1 goal the 3 points are gone. PSG can just rely on their offensive game and if it doesn't work out one day, the other teams first have to win their games, which isn't always the case. PSG would need a contender who can win ten consecutive games in order to put them under pressure.
With the current attacking options, PSG will still be able to score that many goals, but when they are in doubt about winning and have difficulty breaking into the opponent's goal, they will be very frustrated, resulting in losing full points. Nice is a team like that and I often criticize the performance of their players because they are only able to score one goal and the most fearful thing is that when their opponents can turn things around then they have to be willing to lose full points.
Here Nice really feels the worst compared to PSG because the number of goal productivity is very small and that is not good for a team that is trying to compete for the title. They may have a strong defense and concede few goals, but there are times when they will face difficult phases that make their defensive laxity easy to break down. I think it is very difficult to find a competitor for PSG with conditions like that and still PSG will continue to dominate Ligue 1 regardless of which competitor is close to it.