Sorry, I would have to disagree with the premise of the question. The band would never "fire" Stevie. The band revolves around her now.
But I will play along. If Stevie quit (or got fired), it would be silly to try and replace her. Lindsey and Christine could easily carry the band along. Stevie does not play an instrument so its not like Chris or Lindsey leaving where the band needed their instrument. But I do suppose they could have had auditions for the best cow bell player who always misses the beat. You see, I hear HomerMcvie is an awesome cow bell player but his downfall is his timing. It seems to always be on the beat