You make a valid point. No player taken in the draft is a guarantee.
However, you also have to take into account the teams that are drafting. The Pats have been one of the best teams over the past decade of hitting on guys in the first round. There is usually a good reason a team is picking #6 or #7, and it usually has to do with bad personnel decisions made by that team over the preceding years. For example, the Raiders and Lions have been mainstays in the top 10 over the last decade in the draft. They're personnel department is more likely to miss on a guy in the first round than the Pats personnel department is. IMO, when you have a 1st round pick, and you are getting a crack at one of the best 32 players in the nation, you should almost always be able to find a guy who can come in be a difference maker for years to come if you do your homework and have a good scouting/personnel department. Everyone the Pats have drafted in the first round since Belichick got here has played an important role in the Pats' success.
Seymour, Graham, Warren, Wilfork, Watson, Mankins, Maroney, Meriweather, Mayo.
You can't complain too much about that. The Pats haven't missed in the first round, all of those guys have been good contributors.