I think what really killed this game is Arc Raiders which just plays better, normal art style, way more and timely support and it got insane popularity from way back in beta. And it released just ahead of Marathon. This game had no chance. Not to mention the controversy overt the stealing of art assets, shit beta experiences which made them delay and redesign shit to make it more fun. And really in a tiny niche market anyway. These games are not really that popular when it comes to the general gaming audience.
They should have stuck to what made them what they are. PVE shit, raids, dungeons, and those smooth ass shooter mechanics they perfected. Like, IDK, doble down on D2 (too late) or just make D3 with a fresh start and a better on boarding experience for players which D2 is severely lacking. They had the means to print money but they chose some stupid game in a tiny slice of pie genre and spent a quarter of a billion dollars making it and in the interim, stalling any D2 content which also killed that game too. Shit, go back and make Marathon a PvE shooter game like Destiny.