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.
Olympique Marseille is a club that is expected to compete with PSG after they brought Roberto De Zerbi from the EPL to become their manager. He also bought some good players to build a good side. Sadly they performed below expectations last season.
We are even seeing a poorer display this season they have secured just three points from three games. The club lost against Rennes and Lyon. The game against Rennes was more disappointing because they had the advantage since Aït Boudlal received a red card in the 31st minute. From my observation, PSG might keep dominating the Ligue One.
In theory they have always compete with PSG but in the last decade 1.5 decade PSG simply had no competition and I think they will have no competition for this year aswell. However , like you said Marseille is trying for a couple of years to make a project aiming for long term achievements and at some point the results should start coming in. Until then , they have to be honest and aim for that 2nd place this year to play in UCL again next year but also make a great impression this year too.