There are tons of Democrats they could have ran to generate more momentum than Biden. Heck, Michelle Obama could have ran. Alexandria Ocasio-Cortez would bring out more voters than Biden. Bernie Sanders obviously another great pick.
Michelle Obama is (shouldn’t be, but is) a polarizing figure, especially on the right because she’s both female and black. Putting her as the Dem nominee would likely galvanize and energize the GOP base because heaven forbid a black woman be in the White House.
AOC isn’t old enough yet, and she is considered by many outside of her district as too extreme.
One of the main issues (and IMO, one of the dumbest issues) is age. One of the main complaints from voters is that both Trump and Biden are too old. Bernie Sanders is older than both and has already had multiple runs that didn’t pan out. Sanders would just be considered more of the same if he tried running a third time.
The next pick down would probably be someone like Elizabeth Warren, and Trump would be cramming the “pocohontas” thing down everybody’s throats from day one. There’s also the fact that she’s in her 70s herself, giving more support for the belief that all Dems have are a bunch of out-of-touch retirees during a time when the voting base wants younger people with newer ideas and values.
I’ve said from day one that Biden is going to be looked upon as this generation’s Jimmy Carter: A man who actually achieved a lot but was always looked upon as the “lame duck” president after Nixon/Ford that never got the credit for his accomplishments that he deserves. And that’s pretty much exactly what happened.
AOC isn’t even old enough to run. I am thinking you haven’t thought through your argument as thoroughly as you think you have.