I'm with bajaden on this one.
Why does a drafted college freshman have more upside potential than a drafted college senior? He may be able to show in 3 years when he reaches the age of the comparable senior that he had either more potential or less potential or the same potential.
And in the hypothetical case up above by SACKings384 between Randolph and Thompson, how does that help us in 3 years? Randolph gets a lot better, and Thompson does not. That is but ONE possible outcome. The flaw in the assumption is that Randolph will improve dramatically (surpassing where JT is now) and Thompson will not improve (or improve marginally). So what we see now in JT, is more or less what we get in 3 years. Ludicrous.
In 3 years, looking at Thompson and Randolph, we would have the potential to have one player better than the other, or two roughly equivalent players. Even if that hypothetical case would run out and Randolph has a longer NBA career than Thompson, since he starts 3 years earlier, who give's a rat's behind about what might be when Thompson gets to be 30-32 and Randolph is 27-29? Who is to say when each player might reach his prime (if he reaches one), how long he may enjoy that prime, and then, who cares about 10-12 years from now, for crying out loud? The odds say that a player will not last that long with his drafted team anyway.
There is nothing magical about reaching college senior age versus freshman age in relation to potential upside. For every LeBron, there are a gaggle of extremely early entry NBA busts, so what does that say about potential? Players can become superstars or be out of the league in short order, regardless of what age they begin their NBA careers. There is no established age when NBA players stop "growing", some max out on draft day, some max out in a year or two, some max out in 5 or 6 years, some max out nearer the end of their careers.