Agreed. I think it's less about who Belichick let go (I basically agree with most of the players he let go of offensively), but more of an issue around acquisition. They simply didn't fill those spots with talented players as they had in the past.
I think it's fair to say that perhaps too much of the focus was on the secondary. Look at someone like Jason McCourty and Joejuan, a FA acquisition and high-value draft pick, respectively. If that was instead a talented DT and WR, things may look a bit different right now. With Jason and Joejuan removed from the equation, you still have Gilmore, Jackson, Jones, DMac (and eventually Dugger). No shortage of DB talent even without those two.
I understand the emphasis on getting a deep, athletic secondary to match up with the likes of the Chiefs, but too many eggs have been in that basket. The DB basket hasn't been dropped, the eggs are still in-tact, the secondary is still really freaking good ... but they are so thin at other spots.