[STARWISE]

This article claims incapacitation can't force stepping down:

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There is little precedent for forcing a living senator to stand down against his will.

A predecessor of Mr Johnson, Karl Mundt, continued to hold his Senate seat for three years after a stroke that incapacitated him in 1969, although he was unable to attend Senate sessions.

[The Hound Passer]

This article claims incapacitation can't force stepping down:

True, but you still have to be on the floor of the Senate to vote.