Part of the difficulty with a relegation system in baseball is that in the US most minor league teams are affiliated and rostered by a major league club. Not for the independent leagues, but they are outside the MLB structure. The Nats control Syracuse, Harrisburg, Potomac, . . . Potomac does not keep the players that won the Carolina league and go up to the Eastern League. Harrisburg gets them.
It would be especially tough to do for AAA promotion to MLB. The Yankees would not lose the players at S-WB if they were promoted after winnning the IL. This would certainly impact whether a team promoted quality players to the next level. There would have been a big negative for the NYY leaving montero at S-WB. S-WB could not afford the talent the NYY left there if they had to buy them, and they don't have the right to sell them to get money to field a team on their own.