Obama to succeed Hillary would probably have done better. He did maintain his personal popularity reasonably well as President. I'd expect he'd have done the same as Vice President, though it is an interesting counterfactual.
Biden would likely have been stronger against Trump, but Democrats didn't know the election was going to come down to the rust belt, or what would happen to Beau Biden (his cancer returned in May 20 2015).
The biggest mistake of the Democrats was clearing the path for Hillary, which elevated Sanders.
Had they not 'cleared the path' for Hillary, and say for instance Biden had entered, it would've boosted Sanders' (or Warren's) odds and increased the likelihood of a contested convention even more, which would've weakened the democratic party even further. At least Clinton won an outright majority of the vote. Could you imagine how much more intense the 'rigged' narrative would have been in the context of the inevitably anointed DNC centrist candidate winning the nomination by only receiving a minority of the vote and a minority of the delegates?
This is why I'm skeptical of the maps posted here showing Kasich winning 370+ EVs, that would've been a serious possibility in the scenario I describe, but in the modern scenario, the Democratic party was too well-united for that type of landslide to occur. Obama correctly foresaw that the Sanders/Warren wing of the party would be problematic, which is why he let Hillary have it.
