It wasn't just Marathon; there were at least three other games that were in development until recently. That's probably the answer to why Lightfall and Final Shape failed to meet revenue goals: they were expecting Destiny 2 to bring in enough revenue to fund itself and at least 4 other projects, Marathon among them. To my knowledge, there was also Destiny Payback, Gummy Bears, and at least one other. Only Gummy Bears survived and the team transitioned over to Playstation Studios. In short, management broke the company's back by having too many projects in development at once, with only one game to bring in revenue. That's probably why Sony bought Bungie for such a sum (3.6 Billion) in the first place: the executives overpromised well beyond what the company could handle, and Sony was somehow convinced. You could say they felt that trying to "overdeliver" wouldn't be an issue on the business side of things, even though they caution against it for actual game content.