I would not count out either Biden or Sanders. If Sanders comes back roaring it might even help him; he can use his illness as a departure point for talking health care. Biden has been in public life for decades. He's done a lot of things I don't like but corruption is not one of them. He did not become a millionaire in the Senate, unlike, say, Mitch McConnell. The Trump fans, of course, consider every tweet Holy Writ. If he tweeted the moon is made of green cheese they'd all buy Wonder bread to make moon cheese sandwiches. But they aren't voting in Democratic primaries.
Where I think this would hurt him is that his selling point has been electability. He might not be most exciting but he can beat Trump. Now we can expect a nonstop barrage of "crooked Joe" and chants of "lock him up". The New York Times will dutifully write front page stories about Biden's credibility, mentioning in paragraph 51 that there is no evidence for charges. Four years ago, a majority was actually convinced a pathological liar and lifelong conman was the most honest and trustworthy candidate!
Warren is not surging by default. She has risen steadily every month; as people know more about her they like her more. She has "a plan for that", all in comprehensible language. She has raised a ton of money from small/medium donors. And she has thousands of committed volunteers ready to put in the time to get out the vote. Also, she is first in second choice poll. So as candidates drop out, their supporters will look for an alternative and Warren will get a lot of them. So it's not like Biden & Sanders are self-destructing and she's the only one left.