The love-fest started when he put out the #4 defense in the league that carried an anemic offense to 9 wins. Not when he left, as you attempt to characterize it.
IIRC, the concerns about Schwartz were mostly predicated on running a bland, relatively blitzless 4-3 scheme similar to Wannstedt, with no true DE to play opposite of Mario (this was when Jerry Hughes was used only situationaly the year before, lots of ? about his ability to play the run and set edges). Luckily, his scheme actually fit our personnel like a glove (Career years for close for all of our D-line), we got lucky with a few picks contributing a lot early (Preston Brown, Sammy), Leodis McKelvin had his best year of his career (in large part to Donnie Henderson), and we had very few defensive "needs," with Schwartz.
I wouldn't call not expecting all that "fickle," especially not 1 year removed from one of the worst defenses in Bills history that was schematically similar.