I want to know the interwebs answer.
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343 has many Bungie employees that still wanted to work on Halo. Plus Bungie wanted to make Destiny, so everything works out. And Halo 4 was rushed because of the Xbox One release. Microsoft wanted a Halo game on Xbox One and if they waited with Halo 4 it wouldn't be until at least 2016 that Halo 5 would come to Xbox One. Tl;dr Halo 5 will be the biggest and best Halo game yet. (Besides CE of course!)