I really just don't see it, I don't think it was a bad move.
Hendrix's biggest problem was a lack of height. He was often listed as 6'9 at Alabama, and yet several of the draft sites had him a shade under 6'7. Obviously, that's a bit small to play in the post, and given his mediocre outside jump shot, there was just no obvious niche for him to fit into. Obviously his height wouldn't have changed any had he came back another year, and given our struggles in developing talent under CMG it is also unlikely his outside game would have gotten any better.
And furthermore, it's garbage to think that just playing on a winner would have made anything different. Hell, Mario Chalmers made a national championship-winning shot and he still fell to the second round. Joey Dorsey made the title game, and Mbah a Moute made the Final Four, yet still dropped to the second round. And on the other end of the spectrum, guys from truly terrible teams -- for example, Anthony Randolph -- ended up in the first round. The bottom line is that these scouts know what they are doing, and they know pretty well how to distinguish the difference between great players and merely good players playing on very good teams. Even if we were a contender next year, that wouldn't have seriously helped Hendrix's stock.
At the end of the day, I think people just need to come to terms with the fact that just because you left early and weren't picked in the first round does not necessarily mean you made a bad decision to leave early. The harsh truth of the matter is that Hendrix probably just isn't a good enough talent to be a first round pick, and that's the sum of it. Guys who don't have the size for the post or very much outside shooting ability don't often get drafted in the first round, and Hendrix only followed that well established tendency.