If MLB wanted to they could probably do as the Twins have and not let high-risk umpires work, but that would turn into quite the fight, especially with someone like West.
It's his risk to take so long as he agrees to follow the protocols. He says he's been "careful when I go to the golf course" but who knows what that really means? Does he still plan to play golf once the season starts? Are the umps going to be provided for - meals, transportation - with the same level of care and isolation as the teams?
Also, does it make sense to rotate a crew around the diamond as is normally done? The home plate umpire is going to be exposed to the most players, by far, so why have three of them in a single series? Why expose the two first basemen to yesterday's home plate umpire for three hours?
MLB's attempt to do all this testing without siphoning away any resources from civilian testing seem be backfiring on them, i.e., it can't be done.