Taken king and forsaken were both like 10 years ago. Destiny 2 has been a slowly dying giant for a loooonng time, I'm not surprised they wanted to move on.Destiny 1 and 2 were brought back to life by the Taken King and Foresaken. This was possible because even though people had stopped playing - lots of people had bought the game.
With Marathon it's different because it never had the player base to start with. Bungie has backed the wrong game.