The group is not complete - even without her.

Melanie C was the only member of the group that was widely regarded to have genuine, natural vocal talent.

But regardless, each member had their own personality and came with their own mini-fanbase. They weren't like Ms Ross & The Supremes and Bey with LaTavia, Farrah and co; they weren't easily replaceable nor forgettable. Hence the dip in sales when Geri left, even though her vocal ability was questionable to say the very least.