I think it's wise. Although Warren is no spring chicken herself, Joe must be aware of his own mortality, and therefore be really thinking about presidential readiness for his running mate. I would prefer to see her as successor to Mnuchin, but I'm not hopeful that we'll see a non-Wall Streeter in that position in the coming years, even though it's exactly what we need.
History should tell us that the VP usually doesn't matter, but then intermittently it matters a lot. LB Johnson, Truman, A Johnson, T Roosevelt, Coolidge, are some examples of VPs who took the presidency in a very different direction from the one people voted for when they were elevated to the presidency, some for the better and some for the worse but unarguably different. I think Joe is pretty aware of that, and (a) doesn't want a VP who's trying to upstage him, and (b) wants a policy agenda that will have some continuity in case of his demise. Trump harbors no such feelings about his eventual death (or about anything?), and therefore picked the guy who some consultants told him would help get him votes and only ever show complete and total fealty.