I don't think that even with people who really hate the left it's being talked about enough what the hell even is the plan with Biden. Anyone with a brain can see the guy is on his last legs literally, barely able to walk, obvious cognitive problems, clearly hopped up on all kinds of drugs to be able to be in that state in the first place.
Trump is actually right to call him on this and demand a drug test for the debate and I wouldn't be shocked if they simply called it off pretending they're not going to give a platform for 'Trump's hateful rhetoric'. So what if he drops dead? Hell, they could fuck with the election win and then even after they get everything they wanted Biden could still keel over in his second term. Where does that leave everything? I know there are obviously democrats you've never heard of waiting in the wings for that exact sort of thing to happen but first they'd have to contend with Kamala Harris and even in the left they hate her with a passion.
I'm mainly thinking about things like procedure and everything, isn't it the secretary of agriculture next in line after VP? They would potentially have to call elections for a second time purely due to the system utterly failing.
If he drops dead (or drops out) before the election the DNC gets to pick who replaces him. That's where all the speculation about Big Mike taking his place comes from.
There's a clear order of presidential succession, so this isn't a problem. The Speaker of the House comes after the VP, then the president pro tempore of the Senate, then the Cabinet secretaries in order of their department's creation. The Secretary of Agriculture is pretty far down that list.
Realistically it won't go beyond Kackles. If Grandpa ShitsInPants keels over in office she becomes president, and they can't remove her short of impeachment. The president can also fill a VP vacancy, so it's highly unlikely succession goes beyond the VP. It's different if she's just a nominee, because then the DNC calls the shots as I mentioned above.
This might be their move. They have to endorse Biden now because if they didn't it would should they either no longer have faith or never had faith in the The Most Popular President of All Time. The left are a prideful group and will never admit they were wrong.
But if Biden dies suddenly, well, like you said the DNC can pick anyone they want.