Folks I wouldn’t put it past the man to have a “register of party affiliation change” sitting next to him....or already signed, to be dated the day before he passes.
Arizona law apparently says a governor must replace a vacant Senate seat with someone from the same registered party as the previous occupant.
McCain seems to be that kind of person.
Then Ducey huddles with the person he wants and has them change their party affiliation to what McCain changed to.
No sweat. Just have the fire-breathing libertarian you’ve picked out as his replacement/damage control change his registration to (D) and appoint him. Preferably while McStain is alive so he can see his last betrayal turn into but one more in a long list of failures.