He is the only one that could beat Trump. Everyone knew that. None of the other freaks of nature (Kamala, DeBlasio, Buttygig, Warren, Klobuchar, Tom K.) could have beaten Trump because of their weirdness. Biden compared to them looked like a normal, if senile guy. The others were flat out non-starters.
America takes comfort in an old white man. If it was woman or a black man, there would be more resistance to the policies by the administration in power.
Joe looked like a loser too, well into the primaries. I think your assessment is nearly correct - Biden was the only candidate that they could use as a call-back to Obama... and therefore the only one that could have even a smidgen of plausibility after the vote fraud kicked in. That is all they needed.