Depth is a good thing, but that doesn’t necessarily mean burying Mayer in the minors where he starts blocking those coming up behind him.
If Bregman doesn’t come back, Mayer is the best internal option for 3b. Trevor Story compared him to a Nolan Arenado, which is high praise. (Although Story might also have been motivated to keep Mayer at 3b for obvious reasons.)
Mayer could certainly get injured again, although maybe surgery finally corrects this problem. But the Sox could backup him up internally (Romy?), from below (Romero?) or with a multipurpose free agent (Luis Rengifo?) that also fills other needs. Or, if they tender arbitration to Nate Lowe, trade Casas for a candidate. (Also an option in the unlikely event the Sox put a Polar Bear at 1b.)