>If he was being pushed to run just to secure a seat and not because of any personal ambition or sense of service, wouldn’t he have already retired and allowed the governor to choose a replacement?

No that would be an absolute disaster. The governor doesn't choose a replacement for the next 6 years. They just select a replacement until a special election this November. It would give the GOP a do-over with a better candidate against what would likely be Lamb who already lost the primary.

With Sinema leaving the Democratic party that means the Democrats could theoretically lose control of the Senate (should Sinema change who she caucuses with) in a year that wouldn't normally be possible. That would be mayhem for Biden's reelection campaign and absolute chaos if a liberal Supreme Court Justice died or retired next year.