Your logic is faulty.
Situation A -- Player is 22 and not a superstar
-- we absolutely KNOW he has not become a superstar from ages 19-22
Situation B -- player is 19 and not a superstar
-- we do NOT know whether he will become a superstar between ages 19-22
Given that ANY chance is better than NO chance situation B is absolutely, 100% unarguably superior if your goal is to draft a superstar. If the Situation B player does not become a sueprstar by age 22, he is STILL no worse off than than the 22 yr old who we already KNOW has failed to do so.
Situation A -- Player is 22 and not a superstar
-- we absolutely KNOW he has not become a superstar from ages 19-22
Situation B -- player is 19 and not a superstar
-- we do NOT know whether he will become a superstar between ages 19-22
Given that ANY chance is better than NO chance situation B is absolutely, 100% unarguably superior if your goal is to draft a superstar. If the Situation B player does not become a sueprstar by age 22, he is STILL no worse off than than the 22 yr old who we already KNOW has failed to do so.
But the question on the table is whether a drafted college freshman or a college senior has more upside potential? So, why can't the 21-year old move along at the same pace or a greater pace after joining his NBA team than a college freshman and ultimately achieve a higher upside? If players typically reach their "primes" by late 20's, why should we, as fans of our team, care that an 18-year old with roughly equivalent skills to a 21-year old might be better (but not necessarily) when comparing stats at the same age level?
If we knew all players would remain with their drafting NBA teams for their entire careers, this may be more of an issue, but since that is clearly not the case, I just don't see the relevance.