I don't think he genuinely has dementia. I think he's just old and out of fucks to give. A lot of people can't deal with that kind of pressure.
As much as I want Bernie to win, the deck was clearly stacked against him from the beginning. Even if Biden were to keel over and have a stroke tomorrow, you can bet your ass the DNC would find a way around it. There is no hope in hell Bernie will ever be the nominee - the money just isn't there.