He was probably done after Super Tuesday last time (and definitely done after New York) but he clung on until the bitter end.
The nightmare scenario is one where they get to the convention and it's, say, 45% Biden, 30% Warren, 25% Sanders. Then some people will be arguing that it should be Biden because he got a clear plurality, and some will be arguing that it should be Warren because the progressive candidates got a majority (even if it isn't clear that all of the Bernie people would have had Warren as a second choice, or vice versa). Even more fun if the Sanders crowd does mental gymnastics to argue that it should be Bernie in this scenario!
(You could also have a similar scenario with Warren beating Biden and Harris, but not Biden and Harris combined. That one's seeming a lot less likely at the moment, though). In any case, something like that could end up with a lot of people staying home again which would be really bad.
This is why the GOP uses winner take all by state. In this scenario in the GOP Biden would have 70+ and there would be no doubt. ---
September 1, 2003; November 4, 2007; September 2, 2013 Congratulations to DP Oblivion in the Guru Contest!