It's a good replacement; he will serve through at least the end of this year. he may step down then and another replacement would be appointed. In that scenario, we avoid Cindy McCain being appointed right after McCain's death and thwarting the Kavanaugh nomination. We'll also know by then the makeup of the Senate for the next 2 years; if all goes to plan, he'll be able to step down, Ducey can appoint whomever he wants, and it wont make a difference.