I don't think you'd even need to make freshmen ineligible to accomplish that though (and if you did, everyone would just start turning pro). Just make it an inflection point like baseball:
- You want to go straight to the NBA after high school? Fine.
- You decide to go to college? Great, but you're not allowed to be drafted for 2-3 years now.
I realize the above is a league decision moreso than an NCAA / amateur athletics decision but IMO would be the optimal outcome.