I think that Clinton's strategists wanted her to pick a WM to normalize her ticket, just as Biden did for Obama. Putting Warren wouldn't have helped; the real problems (IMO) were the Bill Clinton baggage, neglect of Rust Belt during campaign, and gerrymandering from 2010.
Would Democrats who stayed home have come out for Warren? Don't know. My feeling is that the presence of Warren wouldn't have cleaned up Clinton's ticket, but Clinton would have just dragged Warren down with her...
On the other hand, I agree that putting Sanders on the ticket could have provided the edge that Clinton needed. Not that I was a big fan of Sanders either, but others were!
Ah well, would've could've should've.