You can't just remove a sitting president. He has his people in position at the top of the party.
The Dems aren't going to replace him, not only because of that, but because then Kamala would have to be the nominee. They could not engineer it for a white dude to ascend over her. Reminder: conor and many others defended basing the VP choice for a feeble old man upon race and gender. This is what they get. That part is delicious.