I don't think any person (those who want him out) deny the brilliance and glory/success he brought the club, simply that it is time for him to move on.
I would argue that we were not “shockingly bad” before he arrived as Rioch had already improved the club from the Hillier/Selley period you correctly refer to (a period which still brought domestic and European cup success).
He improved the club, no person will every dispute that.
Danny Fizsman arguably improved the club as much as Wenger did around the period. Nobody ever remembers Danny in this way.