The following analysis is tongue-in-cheek and rooted in pop psychology. Do not mistake this for a psychological analysis proper.
Fandom being Serious Business, it is not at all uncommon for fans to become emotionally attached to fictional characters. As such, when a character departs from the show, whether cruel, abrupt, or even with proper closure, it can cause strong feelings of emptiness akin to a break-up. The process of a new character entering fans' lives can thus be explained through the popular Five Stages of Grief. Actual level of emotional response may vary from person to person. This is a representation of particularly extreme cases (because it's more fun that way).
Caveat (reposted from the main page): Of course, being a Replacement Scrappy does not preclude also being a regular Scrappy. It is entirely possible that the audience would have hated the character anyway.
- Denial: "[Old character]'s not really gone. (S)he can't be! I'm sure (s)he'll reappear in an episode or three. This minor arbitrary line of dialogue proves it!"
- Anger: "I hate you [new character]! All you do is get in the way! What nerve the producers have to think you could ever replace [old character]!"
- Bargaining: "I'll sign this obscure Internet petition and boycott the show! That'll show them!"
- Depression: "Everybody betray me. I fed up with this show! Why are you idiots still watching it? It's terrible without [old character]."
- Acceptancenote : "Hey, maybe [new character] isn't so bad after all! I think I'm actually warming up to him/her! Besides, s/he's just going to get replaced again anyway." Alternatively, if the character would've been a regular Scrappy anyway: "Oh well. [New character] sucks, but there's still a lot to like about the show. Besides, s/he's hopefully going to get replaced again anyway."