Microsoft’s choice was either keep the Bungie name and every employee left to a new studio or they let Bungie go for two contracted games (3 and Reach, people tend to forget Bungie was officially separated from MS before 3’s release).
You don’t have to go to business college to figure out which one was better for both sides of it.
Also, MS kept killing off their non Halo projects.
Marty says a lot of things.
That wasn’t the reason, and Destiny didn’t start as a sci fi first person shooter anyway. It was a medieval fantasy game, hence all the you know… medieval fantasy naming scheme for things that carried over to final release.