Biden, if elected, will not be acting president, due to his diminished mental state. He will be a ceremonial president who signs bills that he cannot read. Fake news is already being trained for the illusion. Sleazy handlers will make Biden a puppet president, wth his VP choice groomed to be presidential puppet number two. The separation of powers will be gone. If you think the collusion delusion was bad and sleazy, wait until a group of unelected officials are in charge behind the scenes. Their paranoia and intrigue will make them dangerous and oppressive.
That being said, Biden's and the Democrats best choice would be Bernie Sanders. That choice would unite the Democrats. If Bernie needed to replace Biden, due to Biden's ever diminished state, Bernie would be a capable person, who cares about the country. He is someone who was nominated by the people. He could deal with the handlers, from day one, and maintain tradition.
All the other choices will alienate Bernie supporters, and will lead to bureaucrats and donors trying to run things behind the scenes. All you need to do is look at the various Democrat governors, who gained extras power via by the Corona Virus excuse; irrational power grab to micromanage adult lives. Biden and Bernie would have earned their power through years of service and hard work, and would be more prepared to use that power constructively and wisely.
However, Trump is still very strong and the collusion delusion scam has not yet to be fully exposed for all to see. Fake news is trying to do damage control. However, this is coming. This will give the America people a vision of the past and a possible future. This vision will make it harder for all the Democrat combinations, who appear to be led by handlers. Even Obama was seduced by his intelligence handlers, and Obama was of sound mind. Biden and Bernie will the best choice, but even this may not be enough to overcome the collusion delusion exposure. Trump was a victim and this will play well eve to blue collar Democrats.