I think the reason Inbox is getting killed is largely because the developers for it are already back working on Gmail - seems after the app got a good reception they we're pretty quickly brought back in.

It's a shame really - IMO it would have made sense to keep developing the inbox concept, it is a quite different way of thinking about email, and if it's a separate app it can move faster than something which has to take into account all the legacy of Gmail. Far from the first time a solid Google product dies just because the dev team moves on to other things and nobody's working on it anymore.