Well, I agree, I've just seen the same thing happen with other bands (great or perfect fit, tours as a guest while they figure out a permanent situation etc.) but they end up picking someone else for the 'permanent' role. Just sayin', Alex has proven a great fill-in drummer for other bands (Timo Tolkki, Stratovarius, Timo's Symfonia, Masterplan, all fellow power metal outfits) whereas Kai's background isn't so Nightwish-oriented.
Of course nothing wrong with Kai either, and it would give him something to do between the generation-long gaps between Wintersun albums, I'm just playing with future possibilities is all
EDIT: Didn't realize Kai has been playing with Nightwish for three years now. Jesus, time flies. Nevermind