A lot of the people who made Halo/Marathon/Pathways into Darkness/Myth either went on to other projects or continued Halo with 343. I would be surprised of most of the orignal crew are still with Bungie, as we have already lost Joseph Staten and Marty (I have not heard about Jason Jones in a while). Bungie wanted new things, so they made Destiny. They split from Microsoft. Ten years only working on Halo would be tiring. Just look at Call of Duty. It's limping towards eventual death. Bungie did not want to do that, so they wanted change