A senator who has that seat locked until 2028 and then mostly like locked again if he should decide to run.
Any replacement would serve until 2026 and then have to run in a special election and then have to run again in 2028. That makes 3 straight elections (7 if you count 2016 (McCain), 2018 (Sinema), 2020 (Kelly special election), 2022 (Kelly regular election)). Arizona is too iffy and too expensive to continue to fund Senate elections every two years.
Beyond that one of the hallmarks of Biden-Harris has been their rock solid support for labor. Senator Kelly voted against the PRO act (something even Manchin voted for) and because of that it died in the Senate. In the last couple of days he has since signaled his support for labor but unions are rightfully leery.
