Why not?

I mean, it's immoral, but looking at history, forced assimilation has succeeded many times - it's just that it takes a lot of time (century+, certainly more than any individual's lifetime) and when it does, we now think of the assimilated people as an inherent part of the assimilating nation. The currently visible examples are just exceptions, either the ones where it has failed as the "assimilating empire" became weak and assimilation stopped/reversed, or the cases where it's recent and ongoing and won't succeed until after I'm dead (but that doesn't mean it won't succeed).