Honestly, how many players would get drafted straight out of high school? Maybe 6-8 a year? If you're not a projected lottery pick you're going to enroll in school. Then, if you are a borderline stud that didn't get drafted out of HS and you don't make a quantum leap your freshman year, you probably come back for a second year, maybe a third year, possibly a fourth. The effect on the overall college game IMO would be minimal other than teams like UK who would have to recruit multi-year players like everyone else. On the plus side, fewer freshman studs means more upperclassmen stars, which means more years for star players to stick around and become (marketable) household names, which is a good thing for the college game.