Shortly after this happened (last week I think), I saw a few articles saying "Google forces all gmail users to inbox" and "Google Kills Gmail", but I haven't seen any evidence of this, and I really doubt they're going to retire gmail. There's too many people using it, and no reason not to leave both services running if they're using the same backend.
I distinctly remember reading somewhere that Gmail interface was simplified and inbox was a much more powerful client.I cannot find that page right now.
Here's an article about it, though: http://www.indiatimes.com/news/world/google-allegedly-planni...
Here's some more:
http://www.forbes.com/sites/gordonkelly/2015/12/05/google-en...
http://www.techworm.net/2015/12/google-ending-gmail-new-inbo...
http://www.technewstoday.com/27625-inbox-it-dont-gmail-it-sa...