The new button performed a destructive action (because it blocked the recipient from responding to the email thread) without any kind of confirmation. Additionally, it was placed where it was likely to be hit by accident. (Apparently where the "Send & Archive" button used to be, although I don't have that on my Gmail theme.)
In short, they didn't test the damned thing. Even a half-hour of user-testing would have shown how easily the button was pressed by accident, and how devastating an accidental press was.
And again: it doesn't matter if the change is a joke or not, you don't push up a change unless you've done some basic UX work on it.