I believe it was the Japanese leagues that put up the barrier, which is perfectly understandable. They didn't want to lose their most popular players. And still don't, so far as they can help it. The first Japanese player in the major leagues was Masanori Murakami in the 60's, who signed with the Giants when he was 19 years old, before a Japanese team grabbed him. He pitched for a couple of years and was very impressive, then went back to Japan and had a long career. I think a better comparison than the Negro leagues would the high minor leagues at the turn of the century into the 30's. The overall level of play was lower than in the majors, but some of the best players in the game were held there by contract. Ruth would have been a legend if he'd never left the old Orioles, but he wouldn't be in the Hall of Fame. The same with Dimaggio and the Seals.