When you said "here is a quote by Belichick" I assumed it was a quote saying McCourty wasn't a very good corner but he was great at safety... especially considering the three All Pro, two Pro Bowl nods and being named to the All 2010 Decade Patriots team?
Instead there's a quote mentioning how DMC is a smart player, which is exactly why he placed him at Free Safety... to erase mistakes. Not only was he the starting corner, team captain in 2012 but he also the team kick returner.
They acquired Talib so they had the luxury of moving their fastest defensive back to safety, and they needed two considering both starting safeties were hurt. BB has talked about this at length but you can actually read about it on Wikipedia, which doesn't have selective memory suggesting every Belichick draft pick sucks:
"On November 1, 2012, the New England Patriots acquired cornerback Aqib Talib in a trade with the Tampa Bay Buccaneers.Following the trade, it was reported that McCourty would be moving to free safety after struggling at cornerback during the first eight games.[44] His move to safety was also attributed to injuries to safeties Steve Gregory and Patrick Chung and the emergence of Kyle Arrington and Alfonzo Dennard as productive cornerbacks. McCourty immediately assumed the job as the starting free safety, replacing Patrick Chung, who was still recovering from a shoulder injury."
McCourty was
"a failure" was a fan take on a not very good defense to begin with, and a poor one. When a defense or team isn't particularly good, you don't blame the best players but the worst.