If the Dems replace Joe, this will be one of the biggest anti-Democracy acts of the 21st century! The voters nominated Joe onto the ballot already. The voters have spoken. removing him will say to the voters: “We don’t care what the people wanted, we decide and you have to live with it.” Because Democrats are the biggest threat to American Democracy.
They voted for the Biden/Harris ticket. So if Biden can’t do it, Harris gets to run for President. She’ll “finish” what Joe had started. There’s no justification to go with anybody else this late in the game. And like you said, it’s not Democratic.