Guitarists are going to be mad, but I'll say it. The lead vocalist is the band. Every other member of the group can be replaced, and you'll still have the band. If you replace the singer, you now just have a cover band. Linkin Park is the perfect example. Chester had an amazingly powerful voice. They replaced him with a woman. It's not that she has a bad voice, it's simply not the same band anymore.
The same could be said about replacing David Lee Roth with Sammy Hagar. It was an entirely different band. If the singer leaves, start a new band. The original one is gone.