By staying in office past May 30, 2018, John McCain has successfully foreclosed the possibility of a special election for his seat in 2018, should he die or resign. Instead, the Republican Governor of Arizona will appoint his replacement and that replacement will hold office until the Arizona general election in 2020.
Very important, as it could have greatly increased the possibility of Republicans losing BOTH seats. The danger would have been that either Arpaio or Ward would have switched to the other race, increasing both their chances and decreasing Republican chances of holding the seats.
This development takes McCain’s seat off the table completely and raises the odds for Republicans for Flake’s seat.