His running and trying to win is to avoid being prosecuted concerning Ukraine.
My theory is close to yours. I think the Dems ended up with Biden because of the Ukraine impeachment.
1. The real purpose of the impeachment was to get a court to give them Mueller’s grand jury materials and raw intel.
2. To do that, they needed to claim that those materials would be relevant to the Ukraine impeachment inquiry, so they had to connect the two.
3. Claiming that Trump was investigating the previous administration wouldn’t be much of a connection, so they had to claim he was pushing the investigation to interfere in the 2020 election.
4. To make that claim they had to claim that Biden was Trump’s “presumptive opponent” because he was the “presumptive nominee” which he most certainly was not. In fact, hardly anyone notable had even endorsed him by then.
5. But by repeating the mantra “presumptive opponent” early and often they basically blocked out any serious contender.
6. Who runs against the “presumptive nominee”? Cranks and ideologues.
7. So at a certain point those were the only two choices: Biden or a crank, and the party was then forced to swallow hard and accept Biden.