As others have pointed out (beat me to it...) this CNN article brings new substance to the conversation. Granted, there has been a lot of talk about this amongst the Dem party and those in his orbit, but the tone and substance of those discussions seem to have shifted.
I think Joe must run (and win) in 2024, at which point he could then at some stage resign, having secured the White House for another four years. All of this is of course predicated upon an impressive current term, for which I am cautiously optimistic.