I doubt that would happen, I can see why people may think like that but I really doubt that will happen. I think it's clear that we are going to probably see how things will change, and PSG will start to win, they are not a team that is used to losing points for too many games in a row, they will eventually get that win. Obviously bigger shocks do happen in football, look at City and how they failed to win for so long, I mean they have just one win out of 7 games or something like that, if PSG has a period like that too, then yeah you are right Marseille could pass PSG in that situation.
But premier league is insanely competitive, even Aston Villa may win Ligue 1, or at least would be fighting for a title, whereas Ligue 1 is simple, hence why they call it farmers league. I would say PSG will not have that kind of bad stretch, they had two ties, which is enough, they will probably go back to winning now, and will not let Marseille get any closer at all.
You have mentioned a valid point about PSG and French Ligue 1 with as things are going most chances we will have some close fights in future rounds as in 15th round we have few big games which will give better view about the future of this league PSG hosting Lyon and Marseille will host Lille both encounters are going to be tough and surely going to be interesting.
French League is surely farmers league due to lack of funds for many clubs and quality players as they needed good investment which will increase clubs stability, but situation is not favourable for having huge investments which is surely important for any league we will have few teams could be created problems for PSG while they are also having not good time but here we need things on well organized for them if they need to keep going for top flight.