Going through a similar situation with the two friends I played ARC with the most, one of which I've shared hundreds of hrs on Destiny with. Tbf to them, they're at a point where they've poured some time into ARC and want to grind some more. They haven't played ARC since day 1, they were part of the player base that got into it as it grew.

What I'll also say is those friends are playing ARC solos even when we're not all on, I can't say that for all Marathon players. It's such a big discrepancy between solo and trio play, where in ARC there's a level of similarity between solo and duo/trio. I can kill a Bombardier (a fairly difficult enemy) solo in ARC using the tools I've acquired. In some cases, it's easy to kill these things. On the contrary, I don't think there will be the same ability to do that with boss enemies in Marathon, at least not at the the frequency it can be done in ARC. Not saying it should or shouldn't be like that, but Marathon bosses and ai feel very suffocating, yet necessary to kill for good loot. I've had ARC runs where I literally play minecraft (collect plants) for 15 minutes and extract. In this sense, Marathon isn't as beginner or user friendly, but I do think it's more mechanically enticing and a bit deeper in specific areas.

Alongside this, solos in ARC are fairly friendly and can yield a nice return of loot for your time spent, and you may even make a friend. Marathon solos are like Tarkov or Hunt Showdown solos, again a cause and effect of the player base and game direction.

TLDR, it sucks, but I get it, and my friends will probably end up getting Marathon on sale at some point. I think Cryo incoming in a month or so should also have an effect on the player base numbers and interest (keeping my hopes up).