Marathon will be a stepping stone for D3
Marathon is starting to reach that 10k player count, and when it goes sub 10k, it's head will be on the chopping block, and I am predicting the following is going to happen.
Bungie will announce support for Marathon will slow down as they restructure and focus on making a Destiny 3 with expansions packs for Destiny 2 to hold players over while building up to the eventual Destiny 3. They will release all currently finished or worked on updates to Marathon to finish up what they started.
After the final update, marathon will be announced to go end of service, and then the following D2 expansion will actually reuse current Marathon assets and include the Marathon ship as a new location for D2, and the Cryo Archive will be restructured as a new raid for D2.
Then Destiny 3 will be released and Marathon will be a simple location, dungeon, raid and weapons set for D3, and the game will either be the savior or destroyer of Bungie as a studio.
EDIT: I want to make something perfectly clear, I am not asking, nor really want a Destiny 3, I am making a predicition on how I think Bungie is going to try and save their studio from closure WHEN Sony decides to cut all support. I'm not hoping for another Destiny, I already served my time, I don't need another stint into that prison. This is merely a predication in what Bungie might do to stave off shut down.