I think Belichick needs to be challenged by guys who haven't been his underlings for the last twenty years. This is true of every great CEO or leader.
When Scott Pioli left in 2009, it was after their drafts had been fairly barren since his glory days of 2000-04.
I just can't imagine that it's coincidental that Reese was there from 2009-12, and during his time, they drafted Edelman, Chung, Vollmer, Gronkowski, Hernandez, McCourty, Solder, Vereen, Cannon, Hightower, Jones, and Ebner. That's basically the entire core off the 2010s dynasty besides Brady. When Reese left in 2013, the drafting results dramatically regressed again.
If it's a coincidence, it's a damn big one. I'm sure there's quite a bit of luck involved, as it's the draft, but we're also looking at some pretty big sample size periods with Pioli, Reese, and Caserio being the main guys in charge. I just think it's a bad idea for any organization to lack opposition voices; and I don't mean that no one constructively challenges Belichick or Caserio; I mean that they have all been influenced by Belichick so much that it's really hard to actually think outside the box when you were educated in the box.