Simply put: It doesn't make sense to invest that much, logically, in today's offense-driven league, into a single position on defense.
This is part of the problem here:
The Ryan defensive system, without the type of Pro-Bowl defensive personnel that the Ravens used to have, essentially requires Pro Bowl CB play.
Jury's out on Idzik, but looking at this roster, this year's draft, and the cap situation, moving one of the two best players on the roster overall (Cro or Revis) who play the same position to upgrade what is an absolutely horrendous offensive personnel group would be a great move, especially considering that Rex is likely gone after this year.
Even if Thurman is retained post-Rex, they simply cannot make Revis the highest paid defensive player in the league without trading Cro. Because I think that's what it's going to take.
I really like Revis. I think he's a Hall of Famer provided his leg gets back to 100%. But it's stupid to look at today's league and not realize you need to be able to outscore your opponent.
Not to mention that you play the Pats twice a year in the regular season. Your investment should be first and foremost in the ability to put up 30+ points.