Ok.... but doesn't the DNC convention (and I'm way out of my sphere here).... but don't they have the ability to chose whoever they want if Joe bails out?
Yes, they do but what they do depends on how much time they have until the election.
If Joe dropped out within the next month, they might just decide to open up an informal primary and tell candidates that they should run campaigns if they want for the next few months and that delegates from all the states can vote at the convention.
But if he drops out late, after the convention, and, for example, after he is the nominee then it gets more complicated. The DNC would have to figure out who is interested and how the freed-up delegates would regroup. etc, etc. Just sayin if he dropped out Lyndon Johnson style and timing they could just hold a basic rejiggered primary and see who wants to run. Later than that, more complicated.