A couple of thoughts:
There will always be OADs. There will always be a set of players who believe, if they cannot get drafted directly out of high school, denied either by fiat or evaluation, then all that they do need is one year of college competition. OADs are here to stay. There will also always be undrafted OADs.
Most scholarship agreements are for just a single year. The school is not obligated to finance a four year degree. The terms of the boilerplate NLI favor the schools, too. It is binding on the athlete but not the school. (Why any elite recruit signs one is beyond me.)
If viewer demand for the games of programs such as Overtime Elite and G League Ignite grows enough, this will all become moot. They will then generate enough revenue to pay serious salaries, and there will always be those prospects who are totally uninterested in attending classes. Additionally, if that viewer demand grows, it will also mean that media exposure has increased, making those programs even more appealing.
Lastly, it is also worth remembering that the NBA draft only has two rounds (60 picks, most years). There is also an inherent risk associated with drafting a player directly out of high school. It seems a bit unlikely that ten to 20 prep players would be picked in any given year. Five to ten seems a bit more likely.