IMO college sports has always been about the front of the jersey (or helmet) and not the back. Hardly anyone watches the D/G league or minor league minor league hockey/baseball for that matter. Even if 50% or more of the top 150 guys go to that league there will still be competitive balance in college which is more important than having the one and done's. I don't think that the popularity of college athletics would suffer much with the development of these types of leagues.
Will the college product suffer ? Maybe or maybe not. The product might get better as more teams will have cohesion and familiarity with each other as they are assembled with tradition freshman, sophomore, junior and senior classes. A better chance for player or team villains and heroes. This could help popularity as well. Perhaps player development would improve as well.
Here's the problem. Nobody is going to watch 18 year old kids play basketball without some brand name affiliation. College basketball is like 10 times worse than the G League but its ratings are 10 times better, why??? Brand name teams who fans associate with