I think you will see more upperclassmen in the draft going forward. Prior to NIL, the only incentive for top players was to start making money in the NBA as quickly as possible. Now, with NIL, there is an incentive to stay in school, especially if a player is not a guaranteed Top 20 pick. Leading scorers on middle of the pack college teams will make meaningful NIL money. Deep NBA bench and G League players do not get endorsements. Even a player that is a likely first round pick but not a guarantee will have to at least consider staying. And depending on how NIL works out at some of the bigger programs, some of these players will be making well into the 6 figures in college. The guaranteed contracts for end of first round players are only about $3.5 million.
A college kids' lifetime earnings could be maximized by staying in school, maxing out his college NIL, coming to the NBA polished and ready to contribute immediately as a higher round pick, earning a bigger rookie contract. The kid might not make it past the rookie contract, but even then, he will make a lot more than he would have jumping to the pros after his freshman year, and being out of the league by 21. Let's say a star at Creighton or Marquette can get $500k of NIL money. That is a lot to give up for the chance of getting drafted.