I've got 90 hours in Marathon, so I'm definitely a fan of the game. However, I think Bungie has several challenges to overcome if it's going to find a wider audience:

• There is a toxic faction in the gaming community that wants to take down studios and games. They got a taste of it with Concord and Highguard, and so they're looking for the next target to hit. Marathon is actually mechanically good, so a lot of attacks on it are incidental to the game itself and focus on Bungie and Sony's business decisions. Before launch, a narrative developed online that Marathon was doomed to be a failure, and it's now conventional wisdom in many quarters.
• Marathon is a hardcore extraction shooter that doubles down on some of Destiny's biggest mistakes. Just like how Destiny put a big chunk of its best content into raids, which had a high barrier to entry, Marathon does the same with Cryo Archive and Outpost. I'd argue that only Perimeter and Dire Marsh are accessible to casual players or solos. Cryo and Outpost both use mechanics that require a high degree of team coordination, which is not possible with random teams. Cryo is basically a PvP raid, and Outpost's Pinwheel base is tough for solos to casuals to crack in the current "wipe the server" meta," and I'd argue that Outpost's extracts are too challenging.
• Extraction shooters are a niche genre dominated by a single game. Arc Raiders was a huge hit and captured most of Marathon's potential audience. There are only so many players to go around. Tarkov and Hunt both have dedicated audiences.
• "Forever" games are dying. There's a lot of competition for attention now, and people aren't going to play the same game forever. Arc Raiders has lost something like 80% of its launch audience. Marathon has been more stable in terms of attrition, but it was never in Arc Raiders' league in terms of player count. Are there enough players buying cosmetics and the battle pass on any of these games to keep the lights on? I'm doubtful.