This is being paired with an international draft, which I think most objective observers think is a good idea. A lot of shady things happen right now when MLB teams go recruiting in Latin America and the calls to do something are growing.
Banning prep players from the draft is a wicked move that is obviously just about further gutting the minor leagues. But the people screwed by this (absolutely phenomenal prospects who can make MLB at age 19 who are now hitting MLB free agency in their late 20's and not their mid 20's) are a relatively small part of the union so I could see this being agreed to.