Why did Lindsey need to be replaced?

[secret love]

Why did Lindsey need to be replaced? After Christine left, they toured as a foursome and just didn't sing her songs. Brett Tuggle filled in on keys. A backing musician could easily step up to play guitar in Lindsey's place and they could just play Stevie and Christine songs. I think diehard fans would have much more respect for a foursome of all original members. Seriously, why was it necessary to replace Lindsey but not Christine?