Was DMC moved from corner to safety because of his versatility, or because he was terrible at corner?
Jeez! THIS again?
When DMac was drafted in 2010, the Pats were still playing primarily zone-read coverage schemes. DMac put up DROY-worthy stats 7 INT, 17 PD, 2 FF, 1 SK, 73 tackles.
In 2011, the Pats began transitioning to more man/press-man coverages, something that DMac had never done. He struggled with it, but improved over the curse of the season and ended up with 2 INT, 12 PD and 90 tackles.
In 2012, none of the Pats' safeties - Chung, Wilson, Gregory - could play deep safety worth sh it. Deep coverage safety still referred to as "free safety"/FS) requires a solid zone-read skill set, as well as great range. So, they began transitioning DMac to deep safety (where he excelled), and acquired Talib as an experienced man-coverage corner.
At some point in the 2016 season (and maybe even earlier), the Pats began integrating some of the zone/pattern-matching coverage principles that BB and Saban had developed together in Cleveland and that Saban had implemented at 'Bama. Zone/matching schemes require that safeties (FS, SS) to be interchangeable, and often require one or both safeties to function essentially as slot corners. They're basically a zone that morphs to man-coverage for some of the DBs after the snap (and that's a drastic oversimplification).
That worked very well for them in 2016, and especially in the 2nd half of the SB.
In 2017, partly due to the lack of competent (healthy) DL and LB bodies, the Pats coverage schemes were almost entirely zone/matching out of the 3-safety nickel ("Big Nickel") and 4-safety dime (unfortunately relying on Richards as the 4th safety). So, DMac has been playing "out of position" quite a bit.
This has all been discussed at length on this forum for at least a few years, and while I'm sure that there are other posters who know their football much better than I do who will take issue with some of the details in my synopsis, it's been pretty obvious to many forum members for a long time that the whole "McCourty was moved to safety because he sucked at corner" was a superficial analysis from the get-go and has now become a really tired cliche.