Devin is without a doubt one of the best FSs in the league and deserves to be paid as such.
I agree that Mayo was overpaid and that he'd probably need to restructure heading into this year, but none of that has any relevance to McCourty's situation. If you are going to go that route, then the logical argument is that NE should never pay anyone a top contract.
Since I doubt that is what your point really is, the discrepancy lies in your assessment of McCourty as a player. I, for one, vehemently disagree with your "solid" assessment of Devin, "outstanding", "elite" or "fantastic" are far more applicable terms.
Here is the thing, when you make an objective assesment of the Patriots defense, while it played the best it has in a long time and was pretty good this season, it never really was quite the sum of what its percevied parts were.
I mean think about how some of the players are described as:
Revis: Top 2 CB in the League
McCourty: Outstanding, Elite Safety
Browner: Strong Boundary Corner
Wilfork: Psuedo Elite Run Stuffer
Hightower & Collins: 2 of the best and most dynamic LBers in the league
Ninkovich: Clutch well rounded veteran
Jones: Above average pass rusher
Siliga, Chung, Arrington, Ayers, and Branch all useful rotational pieces.
With all of these things you'd expect us to be more like Seattle, but we were 12th in Defensive DVOA. We were questionable against the long ball this year. I think we have some elite talent to build on, but lets not overrate every single player.
I was once like you. After the 2012 season I called McCourty an elite safety and had ET and him 1 and 1a in my eye. In 2013 when he wasn't making the splash plays he did in year 1 at safety I assumed that he was making underappreciated plays that we don't really see, so I went to the coaches tape to find some examples. To be honest after watching enough of it, I was pretty underwhelmed with the actual impact McCourty had on the game. He played solid consistent football, tackled well, and looked more dynamic when walked down in coverage, but as a single high (where he plays most of the time), he's virtually indistinguishable from Harmon.
So now as I look at the young talent the Pats have relative to their cap flexibilty, I realize that the Pats don't have the room to pay multiple guys like elite players on that side of the ball. Which is why for me, I want to make sure the guys who are really adding a tangible amount of WAR like Revis, Hightower, and Collins are the ones who get paid. If McCourty is ok being paid like an above average starter by all means lets do it, but he's not an elite player.
I'm not against paying our guys, but I think we need to be careful. That Mayo contract to me is a good example of who not to overpay. Heady and consistent guys are great, but they can never substitute for true playmakers.