If the Democrats had more credible back-up candidates, I have a feeling Biden would've given up on running already - or maybe not but there'd be a lot more pressure on him to do so.

Kamala Harris obviously doesn't have the popularity to be a viable candidate - I'm still uncertain if the widespread dislike of her is fair or not, but either way, after all this time, it's not going to get much better anymore. Gavin Newsom seems to think he's a contender and sure, he's among the highest profile Dems in the country and may actually have the capacity to do the job, but his disadvantages are far too blatant for him to ever stand a chance, the attack ads would write themselves. So you'd end up with, I don't know, Gretchen Whitmer or someone similar? She might be a decent candidate, but could she step up and run a heavily truncated campaign on short notice like that?

On the Republican side, it looks like Haley plans to keep running, but in the process she's alienating Trumpists (calling them 'conservative Republicans' seems both inaccurate and offensive to actual conservatives) so much that I can't see her getting nominated even if Trump would somehow be unable to run - the only way she becomes president is if Trump somehow loses the support of millions of Trumpists in the next few months and in the past eight years with all the crap he's done, nothing even close to that has ever happened...

That poll from Wisconsin the other day (Haley up 15 points on Biden, in one of the swingiest states in the country, while Trump-Biden is a dead heat) makes pretty clear how awful the two party system is for American democracy. Between the three of them, she'd score the best by far in a general election (and is very likely the most capable of actually handling the job), but she almost certainly won't make it there.