Another good point, Bird. There's a lot of room for the picks (currently projected for 19 and 22) to move in either direction. Still, I can't imagine I'd trade two picks for one undersized ball-dominant shooting guard. I don't say that because I dislike Rivers, I say that because it's a risky position and the Celtics have failed to acquire a new impact big man in years. Players I'd give up both picks for (unless one is 12-16 and the other is 15-18): Lamb, Drummond, Robinson, Sullinger.
To say he could fall to the second round if his name wasn't Rivers is asinine. He was ranked as one of the top players in his high school class. This fact alone made avery bradely a top 20 pick. He played on a Duke team that was very deep at the guard position and average 15pts a game as a freshman. OJ Mayo is a comparable player to Austin and he was the 3rd pick in his draft.
I think you're ignoring the possibility that Rivers was such a highly touted prospect in large part because of his last name. And Bradley was a top 20 pick mostly because that was a weak draft in terms of depth. If Bradley was in this draft, he'd likely be projected as a second rounder.
Big credit for Mayo - that's a good comp. About the same size and similar flaws (not quite a 1 because of questionable ability to create for others, lack of consistent defensive effort, a little short for the 2, ego concerns). But Mayo was a better overall shooter (41% to 36.5% on 3s, 80% to 66% on 3s, but only 44% to 43% difference on overall FGs), scored more (20.7 to 15.5), and a little better in other numbers (4.5 boards, 3.3 assists, 1.5 steals to 3.4, 2.1, 1.0) for an overall PER advantage of 22.3 to 16.85. Different circumstances though in terms of college - Austin at the more structured Duke with better surrounding talent but more limitations on his ability to just do what he wanted, OJ playing at USC which let him do what he wanted while asking him to do everything.
In the end, I think you have to look at Mayo's pro career, which has been okay but a bit underwhelming, and wonder would you trade two first round picks for that. I wouldn't. (Though I would love it if the Cs could pick Mayo up relatively cheap.)