Its not really a fair comparison given how the game has changed since Bradshaw's first 4 years in the league. Its a much more sophisticated league; Bradshaw's stats in the first 4 years were accumulated before the major rules changes that opened up the passing game; the scrutiny and pressure is much greater now given the money paid to #1 overall picks and the real-time media we have today; etc.
If you want to be an optimist, perhaps the better comparison would be Jim Plunkett. He was the #1 pick for a bad team, was thrust into being the starter too soon, got beat up mentally and physically, was released and bounced around and was the back-up to Dan Pastornini in Oakland in 1980 until Pastorini got hurt and Plunkett went on to lead the Raiders to win 2 SB's. Of course, even that was a very different league as we've seen in recent years that the Raider's relatively simple vertical passing game no longer works in today's NFL.
If you want to be a pessimist, those that have pointed to the various other high first round QB busts of recent years have just as strong of an argument, perhaps even more so given that the cases of Leaf, A. Smith, Couch, etc. are more recent, though you can find extenuating circumstances Alex's career to date that could make him different than each of the other cases.
The truth is we don't really know what kind of a QB Smith is going to be. We have not seen him healthy and playing in a meaningful game for almost 2 years and when we did last see him healthy, he was an average, improving, game manager type QB. He also had far less talent around him in 2006 and the beginning of 2007 than he would now both offensively and defensively, much less (hopefully) far better coaching from the Head Coach, Off. Coordinator and/or QB coach than he got after Turner left in the 2006 off-season.
The optimists and pessimists are just going to have to wait until the season starts to find out who's right and who's wrong.