I never used Inbox, but this seems like another sign that Google has moved to the next stage in the corporate life cycle; they don't need a testbed for innovative features because they're no longer in the innovation business.
Just because they've finished with the features Inbox was a testbed for (which I am deeply disappointed by because Inbox is so much better than Gmail as a daily driver) doesn't mean they are abandoning that kind of testing approach and have no further use for it.
