Well obviously the NBA would have to make a concerted effort to get top prospects to the G League. They aren't doing that now, but if they wanted to compete with the NCAA they could put the dollars up to sign these elite HS recruits when they graduate.
But why would they though? Why would the NBA increase it’s labor cost to put on a product that many people won’t watch?
College basketball is a lot like the NFL. Fans don’t really care about the superstar player, they care about the Universities.
You can go to New Jersey right now and ask any New Jersey resident what’s the starting line-up for the Seton Hall basketball team ( nobody will answer), but go to Jersey and say Seton Hall sucks. They will drag your ass back to NYC blooded and bruised.
Paying these prospects to go to the G-League won’t serve the NBA in anyway.