I am for the opposite. Raise the age to after their junior season. (or the equivalent if they decide to go overseas if they want to be paid)
The reason, college basketball and overseas is the NBA development leagues. I want to see more NBA ready rookies, not less. I am tired of drafts where we don't really know most the names with players that have longer list of skills they need to work on then skills that are already developed. I am sick of watching 1st rounders spend three seasons sitting on a bench before leaving.
As for preventing tanking, the only way to possible lessen it, is to adjust the rules to favor team play over star play. That way, one player will not make all the difference.
So, when you left high school, did you have to wait a certain time period before you got a job and began working, if that was your choice? When I joined the NAVY out of high school, I didn't have to wait. Why can a baseball and hockey player be drafted right out of high school, but the NFL and NBA don't allow it?
There is nothing stopping them from earning money playing basketball. It is just stopping them from playing in the NBA.
Plenty of organizations have requirements that would prevent people from getting a job with them right out of highschool.
And the reason the NHL and MLB are able to do it, is because they have great infrastructure for player development in place, as well as contracts that are conducive to that, so players do not have to develop at the major league level.
This is so much more complicated than you make it out to be. This would involve massive investments by the owners, and would also involve massive give backs by the players associations as far as entry-level contracts are concerned to make happen. It is a major CBA issue, that neither side really wants to get involved with.
The players association would prefer things stay the way they are, where rookies get paid, and are generally in the majors right away, and the owners would rather have more finished products when they start paying them.