Americans, correct me if I'm wrong, but I get the feeling that Biden became a 'plausible' president very soon after Trump won simply because he didn't run in 2016 and so worked as a "he coulda won" figure. If he'd actually run for the candidacy, he'd be squeezed out by Clinton and Sanders and What If Biden Was President would be a tongue-in-cheek AH thing that isn't taken too seriously.
I do need to reread their PM Johnson stuff now we've actually got him.
Joe Biden has been at the very least a possible dark horse candidate for the Democratic nomination in every cycle since 1984, and a plausible enough candidate for the nomination in every cycle since 1988, with the exceptions of 1996, 2000 (where even here I think he's back in possible dark horse territory and is one of the few Democrats who could launch a plausible though very unlikely Please Not Gore) and 2012. I intentionally include 2016 from this; he would have definitely given Clinton a run for her money, and, since both splits the moderates, probably cleans up a healthy chunk of the Obama Loyalists and he probably sweeps up the Please Not Clinton vote instead of Sanders, he has a very good chance of winning the nomination.
The thing is that in 2008 and 1988 he just fucked it up, which meant that it's also quite possible that he just fucks it up in any other primary race he runs in throughout the 1984-2020 period, including this one. And, in terms of inevitability, he didn't seem like the inevitable candidate this year until a surprisingly late date considering that he thoroughly and utterly dominated the 2020 primary process, though in retrospect it was obviously his to lose -- but, again, given his record, that doesn't mean he doesn't lose it.
When Vladimir Putin became Prime Minister in 1999 few expected him to stay long. He was the fifth PM in 18 months and was generally seen as a rather colorless individual. The front-runner to succeed Yeltsin was former PM Yevgeny Primakov. Then the apartment bombings and the Second Chechen War happened. Putin demonstrated that he was a capable leader, and his popularity rose until he was the favorite to succeed Yeltsin.
Considering some of the credible allegations that have come out about that period, there's a distinct possibility that Putin was made to look like a capable leader with intention and forethought. Even if you discount the most extreme allegations, that the apartment bombings were an inside job to trigger a war in Chechnya - which I tend to be somewhat skeptical of myself, though I find myself unable to discount them - and just go with the idea that Putin's role was specifically and intentionally bigged up by Yeltsin and his own men in preparation to try to hand over power to him as part of a deal to prevent their own prosecution by the next administration. Considering how well Putin cleaned up for and covered over his previous boss' sins in St. Petersburg before, the idea of him being brought in by Yeltsin to do exactly that but for him this time is very plausible indeed.
But none of that, of course, changes that he seemed to come completely out of nowhere to being one of the most powerful men on Earth for the next two decades. His eventual ascension to the Russian presidency was probably entirely unforeseeable even four years earlier.