To me this is not a a case of either/or.
Yes, you are correct that Seattle utilized and hit on later draft picks to build their defense.
That is a completely separate topic and argument that (in my opinion), a team that is drafting half a round earlier than another team should be expected to improve while the other team regresses given the parameter of today's NFL (i.e., salary cap; free agency; draft based on inverse order or record).
For 28 consecutive rounds Team A starts out with a draft slot that is half a round earlier than Team B's draft slot. Team A improves; Team B does not. That should not be a surprise; that should be the expected result.
Assuming a straight draft, with no gained, lost or moved picks:
Teams that draft at the end of round one instead of higher up on it are only behind those drafting at the top by one pick. After the poorer team makes its first pick in the draft, the issue goes away as the better team gains the higher picks. The lesser team didn't draft ahead of the better team 28 times, it did so 4 times.
On a total points (just subbing the word "points" in for "slots" scale, excluding compensatory picks, and with 1.1 getting 1 point and 7.32 getting 259 points, the most extreme difference you'll get (1 v. 32) would be 217 points. That's only an average difference of a little more than a 7th round pick (#22 overall in round 6). While the compensatories alter this slightly, the overall difference, while there, is not mind-blowingly significant, which is part of why bad drafting teams tend to keep stinking year after year. When teams blow that first round pick, they blow their advantage and, especially before the new CBA, they actually end up putting themselves at a future financial disadvantage. Don't get me wrong. I'm not saying that it's not an advantage. I'm just noting that it's not as huge an advantage as one might think, particularly given the number of positions teams need to fill.
The key to success in the NFL draft is to hit consistently on top 3 picks, and to occasionally hit big on later round picks. What happened with Seattle is that they hit big with several later round picks in a two year window (2010-2011), while taking advantage of 1 top pick (Earl Thomas) in the 2009-2012 time frame, and busting (Aaron Curry) with the other. They flew past the Patriots defensively not because they were drafting higher in every round, but because the Patriots were drafting Ras-I Dowlings in round 2, while the Seahawks were drafting Richard Sherman in round 5.
In the case of the Patriots/Seahawks defensive comparison, the Patriots actually spent more draft capital on players. They just chose much more poorly, in comparison (ignoring the notion of future improvement solely for the sake of grading out at this very moment).
The Patriots spent 20 draft picks on defensive players from 2009-2012:
Patrick Chung
Ron Brace
Darius Butler
Tyrone McKenzie
Myron Pryor
Darryl Richard
Jermaine Cunningham
Brandon Deaderick
Kade Weston
Ras-I Dowling
Markell Carter
Malcolm Williams
Tavon Wilson
Jake Bequette
Nate Ebner
Brandon Spikes
Devin McCourty
Chandler Jones
Dont'a Hightower
Alfonzo Dennard
To date, not one of them has established himself as a star, the bolded are already gone, and the underlined have failed to establish themselves at all, to this point. Spikes, whom I didn't underline, has established himself solely as a one down (i.e. run stopping) player, limiting his impact, as well. If we're looking for reasons why the Patriots have not become a dominant, Seahawks/49ers type defense despite having players like Mayo/Wilfork (earlier draftees) and Talib (trade) on the roster, those 20 draft picks are a good place to start, rather than pointing to the draft system itself. After all, 10 of those 20 picks were taken in the first or second round, while the Seahawks only drafted 4 defensive players (including the big bust in Curry) in the first two rounds during that same 4 year stretch.
