In fewer words, Inbox started as a UX redesign, was met with so much resistance that the team lead had to write a "You are not the target user" post internally, and to salvage the sunk costs, they released it as a separate interface.

Personally, I hope that Inbox gets some of those currently missing features added back in, starting with support for Google Apps accounts. The ability to snooze emails is too powerful (followup.cc turned it into a business model) to allow it to get thrown into the Google Labs graveyard.