Mitchell, Bennie is most likely happy with his achievements through Biden. The bills that have been passed are really under appreciated (eg. reduced child poverty by a quarter). Disappointment is a natural consequence of promising much more than can actually be delivered.

President Sanders? He most likely would have delivered no more than Biden although he would have been a hell of a lot better on the selling side (which is dreadfully lacking). Sanders is a great communicator and Biden is not.

Going forward Democrats have a difficult road. And Biden for 2024 would very likely lose to a semiskilled Republican opponent. Harris would definitely lose. So this arrangement has to be tended to but certainly not openly till the midterms.

Best to you.