They can go overseas now and the NBA D league could allow them now if they wanted. But if you play you lost your college eligibility. NCAA should not budge on this
Kids can already sign immediately into the G-League. Several have, some even eventually going in the 1st round. The reason it isn't more prevalent is they receive a salary of like ~$30k (although, 100% of travel costs and food is covered and they receive a per diem when on the road), which is far less than almost any D1 athlete receives on the side. Far less than if they went to China.
If I was a blue chip or my kid was a blue chip and he valued my opinion, I'd take the endorsement money from Adidas, Nike, Under Armour, Li-Ning, or Anta and do the $30k salary, receive the 2nd best coaching (from past and future NBA coaches) in the world, compete in the 2nd best league in the world, and aim for that lottery. The reason I think it hasn't caught on yet is China and a few other teams in Europe you see take a lot of US kids reserve 1-2 of their foreign player roster spots for 18 year old Americans, and they're more than comfortable spending 7-figures on 1-year contracts. Hard to objectively look at a 7-figure salary and a $30k salary and say "I WANT TO IMPROVE AS A BASKETBALL PLAYER SO I CAN HELP AN NBA TEAM WHEN I'M 20!!!!"