There are a number of problems with evaluating a draft year in a vacuum:
1) It tends to ignore the overall strength of the class and assumes that every year the talent pool is the same. It's hard to figure this out of course, because you'd have to look at EVERY team and compare their success rate with the Patriots, and I doubt anyone is really willing to dive into that mess.
2) It ignores the state of the team itself going into the draft. Some years are harder than others for rookies to make the roster. So even trying to use a metric like "was he a starter" etc, you have to figure some guys could be starting and not deserve it if the competition was better, or might have started if they weren't blocked by a veteran or surpassed by a surprise hit somewhere else.
3) It assumes that the draft is an independent piece of roster building, when in reality it's heavily connected to all other parts of the offseason (free agency, extensions, trades, etc). You have 90 guys you can invite to camp, and Belichick isn't going to keep a rookie over a better veteran player just to make his draft success rate look better. Once they're in camp, players from ALL sources of team building have a chance to earn a spot. So, related to point 2 above, this creates a situation where sometimes the rookie just isn't in a great spot to contribute. This is NOT a draft failure, but a free agency success, in that specific instance.
So when I see the team reach three straight super bowls, I have to believe that their team building philosophy (which includes, but is not exclusively the draft) is near the top of the league. If you want to dock a few points because they have Brady (which admittedly gives them some leeway in some areas), that's fine, but we've seen what happens when Brady doesn't have a good roster, and it doesn't result in super bowl births, with the possibly exception of 2011 (though that team had good offensive talent, just no one in the secondary).