It's simple. If Kelsay was the EXACT same player, playing the role of a hard-working depth DE who came in and contributed a small handful of sacks on a winning team, he'd be downright beloved.
As a below average starting DE who contributes a small handful of sacks on a losing team, he's reviled. People need to ask themselves, "Is it Chris Kelsay's fault that the Bills took 9 years to find a pair of better pass-rushers?"
And really, only time will tell if Anderson is better.