We're not talking about throwing new people at the product during development, though.

There are aspects of software development that have to be done in series, but content production can be a very parallel process. Also keep in mind that much has been made of the tools being next-gen ready. And that development would have probably been started before Halo 4 launched.

I'm not saying I like the idea of a two-year release cycle, but you can absolutely develop a large, feature-rich AAA FPS in such a scheme.