Bonuses were skyrocketing after WWII, and owners implemented various bonus rules to reign them in. Bonus babies were a result of that, e.g. Al Kaline. So yes, the Yankees and Cardinals were hoarding but it was mostly money because this limited the amount those rich teams could spend. Once the Bonus Baby system fell apart, the draft came in, again completely stripping leverage from the players. Its main goal was to suppress bonuses.