There's been some revisionist history out there, but Inbox was intended to be "the next version of Gmail" at the time, until a near revolt internally about how the Gmail team "ruined Gmail" led them to launch it as a "separate product" instead.

Now that there's a new new version of Gmail, which was rolled out a couple of months ago that hasn't upset everyone, it's unsurprising the unloved stepchild of the family is going away.

Old Gmail was version 1, Inbox was version 2, and the new Gmail is version 3.