Jim Schwartz turned the #10 ranked defense into the #4 ranked defense. The Bills defense was already pretty good under Pettine. People forget that the Bills all-time sack record still stands from the 2013 season, when Pettine was the DC. The Bills already had a good pass defense when Schwartz got here...that didn't change much. What Schwartz helped turn around was the rush defense which, among other things, helped the Bills allow much fewer points. So, yeah, Schwartz did do a great job in his one season in Buffalo.
But if you look at Schwartz's entire body of work, it's not really that impressive...not consistently good by a longshot. For example, in his first season as defensive coordinator in Tennessee, all he did was turn the #1 defense from the previous year and turn it into the #25 defense. He helped take the #2 ranked team in points allowed, and turned it into the #25 team in points allowed...the team gave up almost 200 more points than the year before he got there.
In other words, I wouldn't go about hoisting Jim Schwartz on your shoulders and parade him around the town square just yet. Bad seasons happen, like the time Jim Schwartz's defense was ranked dead last in 2006 in Tennessee, with only one other team allowing more points.
Hell, if you want to justify demonizing Rex Ryan and aggrandizing Jim Schwartz, then compare their entire track records. Compare their results as Head Coaches, defensive Coordinators, and even their defenses when they were Head coaches. I guarantee you that Rex comes out way ahead in just about every category in the end. Here, these two links two links will make that pretty easy to do...one link for Ryan, and the other link for Schwartz:
http://www.pro-football-reference.co...es/SchwJi0.htm
http://www.pro-football-reference.co...es/RyanRe0.htm
Again, I realize that stats aren't the end-all, but c'mon now.