What happens to 18-year-old kids who have no real interest in attending a college or university? The only other option available to them is the independent leagues. It could get really amusing if some can't miss high school kid winds up in the Atlantic League, with a club that holds a multi-year contract with him, and the kid proves to be essentially major-league ready. The kid is drafted as a 19-year-old, and the Atlantic League club demands a large payment (think Japanese player posting fee) in order to sell the contract to the drafting MLB club?
Is MLB saying, "Go to a college or Junior College — not to get a degree, which neither we nor you give a hoot about, but because these schools are now our free (to MLB) player development system."
Then there's also the question of international players. 18-year-old American high school graduates are not draft-eligible, but 18-year-old Dominicans can be signed? they're planning an international draft with eligibility at age 18.
Also, what happens to international player development if there's a draft, and the buscones who shepherd and develop these kids before they're old enough to sign are suddenly cut out of the revenue stream? If the buscones have to find other lines of work, there will simply be less organized youth baseball in places like the Dominican and Venezuela, and less talent developed, period.