Now if I thought he was a future HoF I'd be fine with them holding him back.
As for the bolded, we don't know. It is a possibility that him not being up in September will impede his development. Means learned a lot from his time in the majors in 2018. Same might have been true for Mountcastle. I think it is very poor logic to just assume that he wasn't damaged while simultaneously suggesting it might be helpful.