I'd say that he's been successful in unexpected ways. Has he evolved into the point guard of the future, the heir to Rondo's throne? Unequivocally, no. Has he proven to be a positive presence, one who consistently contributes to winning basketball games? Absolutely. Ultimately, that likely means he's been slightly disappointing, yet still quite valuable and important.
Additionally, some of the declarative statements made by the OP are patently ridiculous. I think I'd rather have Gordon than Smart, which doesn't really reflect poorly on the C's given their draft slots, but the idea that Randle is clearly better (or better at all) than Smart is ludicrous. Randle has been consistently bad to meh on one of the worst teams in the league. And following that theme, I think it's very unfair to call Smart a "missed opportunity." Due to our need for increased scoring on the second unit, I would probably concede that we'd be better off with Zach Lavine than Smart. Gary Harris and Clint Capela are another two that I'd likely take over Smart. However, to draft any of these guys at #6 would have been considered certifiably insane at the time.