He will only leave if he decides to leave. Like him or hate him, he is still the President and has the final say bar a 25th Amendment situation.
The Dems want to avoid a 25th situation at all costs.
That would mean admitting they were lying all along, and it would raise Kamala to the Presidency, which they do not want. They would then have to run her in the General. They couldn’t dump her and lose the women’s and blacks’ votes, even if they don’t like her all that much.
Plus there’s Jill they will have to contend with. She will not go quietly. She enjoys being defacto POTUS.
The Dems are between a rock and a hard place, politically.