Teams cut great players all the time. Could be pricetag, could be team-player relationship, or could be something more obvious: He’s a career 4-3 3-tech and they’re converting to 3-4, so he’s not a scheme fit. He’s a little oversized for a 3-4 DE and he’d be completely wasted as a NT, a la Albert Haynesworth.
These are the last 6 seasons for McCoy, keep in mind he’s a DT:
2013: 9.5 sacks
2014: 8.5 sacks
2015: 8.5
2016: 7
2017: 6
2018: 6
You could say he may not be what he was in 2013, but a run Stuffing specialist who’s a lock for 6 sacks every year is a phenomenal acquisition at DT. He’s still got it and should be a Force well into his 30’s.