Did Talib make THE difference? Signing him allowed Arrington to move to his natural spot of nickel and allowed McCourty to move to safety. When playing Talib covered the other team's top receiver.
And yes, the other part of the difference in the secondary was the ability of Dennard to step up, after Cole stepped up before he was injured.
Belichick has decisions to make at corner. However, four corners are free agents: Talib, Arrington, Cole and Martin. It is extremely difficult to go into free agency and find 2-4 corners.
I'm not exactly sure what you're asking, but I am assuming that you have the same thoughts as me regarding whether or not Talib made 'the' difference in the secondary or not.
Yes, of course we all know that he helped initiate the change for McCourty to safety, although that happened prior to him coming here, and there are many reports that surfaced that claimed that the front office wanted McCourty to be our FS from training camp on, mostly by SMY and Bedard. The "story" was that they decided against the permanent move due to not having enough depth at CB. Take it for what it's worth, it may be true it may not be true, but it has been reported by multiple accounts, and either way Devin McCourty did move to FS before Talib was signed. Of course one could argue that Talib allowed DMcC to remain there, but I believe he would've stayed there anyway.
I also believe that another CB can be had for much lesser money, depending of course on what Belichick wants to do with Arrington, etc.
We can realistically still have McCourty at safety, Arrington in the slot, and Dennard on the outside. The final piece would be selecting another CB/S either in the draft, in FA, or both.
That decision will be made by Belichick, but I doubt he's going to go over about 7--7.5 million with Talib myself.
Keep in mind although we're talking about a guy who has certainly helped the secondary in many ways, he also has played pretty sparingly since he came here and broke up a meager 2 passes defended (and the INT return of course). I realize his potential here, but I also think that he is truly overvalued by many at the same time.
It'll be up to Bill to decide what's best for the future of the team, but I do doubt that he's going to break the bank for him, especially with some question marks about his attitude and character in the past, although he may feel that those issues have been resolved since then too. Time will tell.