Mayer will be AAA to start, deservedly so since he has zero AB's in AAA due to constant injury issues that have slowed his progression. However, this doesn't really leave him out of the picture in the short or long term. In the short term, any number of guys are currently struggling with injuries or at least have a history of them (Devers, Story, Masa, Casas) and others have a history of less than ideal performance (Grissom, Hamilton, Romy). In the longer term, Bregman is not guaranteed to be here next season which could open up a roster spot at some position for him next season. He does have things to work on (offspeed pitches, platoon issues) and could just use a full healthy season at AAA.
Honestly, I think too much is being made out of the small sample sizes we are currently seeing in Spring Training. We've had prospects that weren't really considered ready have good spring showings before (JBJ) come up and have to be quickly sent back down. I think it's good for Mayer to show well and just continue to show well after going straight to AAA.
It is weird to say "he didn't spend much time at 2b in the previous season" when the majority of GS were at 2b (36%). He didn't play 2b at AAA because they needed him to play SS once the big 4 were called up and Mayer went down with an injury. If Mayer hadn't gone down with an injury, I'm sure his starts at 2b would have been dramatically higher.