I think in their mind (at the conclusion of the 2019 debates)...Joe is the only viable candidate to run against Trump and win. I would argue at this point, mayor Pete probably would be a better guy presently on the campaign trail, and no serious dementia or health issues.
I think at the end of the first debate...the whole Joe-campaign is set to fail and the plan B will not be acceptable to them.
I don't know why they didn't go with Bernie. Bernie was the closest to Biden in terms of primary delegates, and the student Red Guard types were enthusiastic about him. Most of the rioters are commie Bernie-bots. It makes no sense to me to coalesce around the allegedly more moderate and senile Biden, especially as he has embraced all of Bernie's positions.