You are very correct and this shows you have been following the French Lig 1 for many years. There was a time I expected Marseille to challenge PSG for the league title but somewhere around the middle of the season, they changed their mind and for reasons best known to them. Consequently, they started losing matches they ought to win and playing draws in some and ended up second in the table. For years to come and if this pattern persist, I don't see any team that will stop PSG from winning the league.
That was precisely last season, Olympique Marseille seemed like the team that could challenge PSG for the ligue 1 title, they had some wonderful players in their squad and they did wonderfully well in the First half of that particular season which made them one of the favourites to lift the trophy but at the long run everything went bad when they started losing important games week after week, and numerous draws thereby giving PSG the opportunity to continue to dominate the league as usual, the rest is history.
It's always been like that every season for the past years in the Ligue 1, different teams in the league upgrade their squad and seem as though they may likely be the team that would take away the trophy from PSG but in one way or the other, PSG still manage to win it and with a very good margin, I wonder if any team would be able to beat them to it this season.
Although I feel several teams would come close this season, Olympique Marseille is one of the strongest and would compete well likewise Monaco, those are the teams I feel might likely try or come close to winning the Ligue 1 title this season.