This is too sensible to deny. Hillary: she couldn't, she can't.
As to Michelle Obama, somebody's wife as a last-minute switch is not unknown but it won't go over well on the presidential level. She'd be a token of her husband and a puppet of the Deep State. Right now she's rich and famous and living it up. Case closed.
I believe (subject to change!) the Dems know they are going to lose unless they can crash the economy and/or destroy DJT. They know it, therefore it doesn't matter who steps up to lose. But by November, something might go their way and then they need the most unobjectionable candidate. That is Biden. He can sway a few more voters where it counts (Rust Belt), as socialists cannot. Besides, he's so old, he's disposable, whereas Warren is still young enough to run again. And she's not all that popular. True, Biden has a lot of baggage (yet to be unpacked), but he's a better liar than Warren. Plus he'll take whatever running mate they tell him to take.
In the end, Trump has a 90+ percent chance of winning and will probably have it on Super Tuesday, so they'll play their deuce. Old Cracked Joe.
Biden has the least to lose by running at this point; he is an older guy, and the corruption involving his son would probably be more widely known by 2024.