I'm not sure I understand what kind of system you're proposing. Amateur players get drafted by MLB teams, and then are under that team's control for a certain number of years. They play in the minors first because in baseball, more than any other sport, it takes time to develop skills. Newly drafted players aren't ready to play in the majors immediately.

The draft, and the years of team control, are important for parity. MLB has more parity than any of the other North American sports leagues, and MUCH more parity than the European soccer leagues. I think that's a good thing.

You seem to be proposing a system where there's no draft and every amateur player just becomes a free agent. So, for example, a guy who would be a top draft pick can instead sign a 1-year contract with a Korean team, and then try to sign on with a major league team the next year? Correct me if I'm misunderstanding.