Yesterday (Wednesday the 20th), Senator McCain was successfully operated on, for the removal of a 2 inch tumor attached to his brain. Doctors believe that small tentacles of this tumor are still in the brain, necessitating chemo-therapy.
Apparently the Senator is reasonably healthy in spite of this most recent set-back.
If his immediate "recovery" allows him to return to the Senate, I do not expect him to retire. He is too much of a #NeverTrump politician. His continued presence on the Senate roll-call, will add to the difficulties for President Trump's agenda and that will be, very much, to the liking of the Senator.
McCain is a Status-Quo, Big government, Tax and Spend, moderate. He has accomplished next to nothing over the past decade, except to superintend a presidential campaign that handed the Dems super-majorities in the House and Senate and gave us our first Rogue President.