The point I'm about to make is a total side-point. It's just a bonus for having players of that caliber.
KAT contributed to Kentucky in the form of being "the guy" on a great defensive team (and a historically good team). Plus, that stuff does pay off in recruiting. When you can point to the best bigs in the NBA, like KAT, AD, and Boogie, and note that every one of those guys came out of UK in a 5 year span, recruits take notice of that, which in turn, contributes to future teams in some manor.
In 3 more years, nobody is going to be saying "I want to go to Duke to be like Grayson Allen", and in 6 or 7 years, nobody's going to go to Virginia saying "I want to go to UVA to be like Kyle Guy"
Hell, if Louisville produced good NBA players (and I absolutely do not mean this as a shot in any way, shape, of form, Louisville fans), then their recruiting would be much better. It's not like Louisville doesn't get these kinds of players for a lack of trying. Pitino recruits them every year, they just can't get those guy because they don't produce really good NBA players.
The main point that needs to be made though, is it better to have Grayson Allen (or Kyle Guy, in the point that you're trying to make) for 3, or is it better to have Karl-Anthony Towns (or we'll go with Devin Booker if we're sticking strictly to 2 guards), then the next year have Jamal Murray, and then the 3rd year have Malik Monk?
Grayson Allen is arguably pretty close to his ceiling. His role is, yet again, about to change. First he was the 7th man or so down the roster, next he was "the guy" on a pretty limited Duke team, and now he has to revert back to being "just another player" on a Duke team that will have a few very capable scorers in the starting line-up, but no facilitators.
He won't match his numbers from this past season. More offensive firepower, and plus he does have to play with the black hole that NASA has decided to name "Jayson Tatum".
Grayson Allen could very well not be as good as Malik Monk, he could very well not be as good Josh Jackson, or Markelle Fultz. Is he the best player from the 2014 class to remain in college basketball now that Ulis has signed an agent and is going to the NBA? Quite possibly, but that definitely doesn't mean that there isn't a younger, better 2 guard out there that is about to come into college basketball.
Right now, as a UK fan, I'm perfectly content with the fact that we got Devin Booker, and then Jamal Murray over 2 years of Grayson Allen.