Have you missed the memo? Google+ is "Google's central identity" in the new Google. Shutting down Google+ would be akin to Facebook shutting down Facebook.
That's just marketing speak. Sure, G+ is about getting a real identity and other personal info to tie together with Google reading your emails, tracking you across the web, and tracking you via Android etc, but a Gmail sign-in would provide a "central identity" without G+. That's especially true for most Android users.
