OK, chickens and eggs, sort of hard to split the performance results between quality of player and quality of coaching.
So let's assume that McCourty, Wheatley, Dowling, Butler, Chung, Wilhite were all poor players and poor draft decisions.
Look for a minute at this pool:
Neal, Connolly, Wendell, Koppen, Hochstein, Levoir, Yates. Throw in more highly drafted players (but considered "reaches" maybe?) in Vollmer & Kaczur.
In their first or second season with the team, you would not have pointed to that pool as top talent. Yet superior coaching and commitment helped them develop into solid players while here. And those that left mostly tailed off when they did.
We've also seen late-round DL do well here (Love, Pryor). Late round RB (BJGE, Bolden potentially). There's a late round QB who has done well.
So, either we are mis-remembering some moments of hope - recall the concern last year when "Chung, the Patriots best player in the secondary" was out, and check out the Dennard thread or the comments about "more Tavon Wilson" - or those players in the secondary never progressed.
Regardless, the secondary players were mostly drafted in the #30-50 overall range, and that similar pool of OL was mostly drafted in the 150-udfa range. According to NFL experts, there was more talent in the secondary group. It just never developed.
In comparison, the Patriots have failed to develop their players in the secondary. Seems to be a long enough track record that there is a trend. And should be a concern.