I would be for a promotion system.worst umps by pitch fx evaluation get demoted to AAA and best AAA umps get promoted to mlb. This could weed out some declining umps and improve ump quality.
Umps did get much better since pitch fx was installed but promotion system would make it even better.
But yes, of course most missed calls are tight "50/50" calls albeit there are some exceptions.
I would be interested in a bad call stat, i.e missed calls by more than 2-3 inches.