Plus, why do people act like the "incumbent advantage" is some magical advantage? It's a cargo cult mentality, especially in this day and age where all the old "rules" about elections have gone out the window.
I mean, I remember the day where being twice-impeached and a convicted felon would be unrecoverable political death, yet here we are staring down the barrel of a possible second Trump term.
Biden is a historically unpopular president, who is behind in basically all polling in basically every key swing state, and who just had the mother of all "the emperor has no clothes" moments on national television, losing the confidence of his own base. Even Democratic congresspeople are calling on him to step down now.
There is simply no path forwards for Biden to win in November. He's cooked.
As for replacements, personally, I think Gretchen Whitmer is the best choice. Relatively young, good compromise candidate between the progressive and moderate wings of the party, current beloved governor of Michigan (key swing state!), competent technocrat, no significant political baggage, and made a name for herself protecting abortion rights in Michigan after SCOTUS overturned Roe v. Wade.
Some squatter who bought the rights to it for two twigs and a raspberry back when they were first selling off name rights 200,000 years ago