The Democrats have a tendency to have a relative unknown show up out of nowhere and suddenly become the party's darling. If President Biden doesn't run, the next candidate could very well be someone you've so far never heard of.
And the GOP's chance depends on whether they win the House now. If they do, maybe they can stall Biden at every turn and then paint him as a do-nothing. If they don't, we could see an avalanche of progressive legislation, especially if Rev. Warnock wins in Georgia next month too. That would be tough for the Republicans to shoo aside.