I should note that, with the R in the 70s (a SD Senator, IIRC) the R caucus, before too long, stripped him of all of rank and priviledges and committee assignments.
But not his seat. If Johnsons disability turns out to be long term then no doubt the Democrats will replace him in any leadership positions he may have but he'll still be a senator. I don't think that the GOP allowed Strom Thurmond anything other than ceremonial duties his last four or five years.