Mathematically, your race will inevitably be replaced and become extinct if you import another widely divergent race with a higher birth rate, such as is the case with almost all imports/invaders in Western countries. Or, if you mix with highly different invaders in significant numbers, you will be made extinct and be replaced by a hybrid population. And either way, since culture is downstream from genetics, your kind and your values and everything your ancestors fought for and embodied will be wiped out.

Now I don't say this to promote petty, childish nationalism. There are many human ethnicities that are compatible with each other, who are nearly genetically, behaviorally, and visually indistinghuishable, and hybridizing with them wouldn't change anything -- but, if we are to be honest, we must admit that some are not compatible and hybridizing with them would change almost everything. If we care about the future of human evolution, we'll care about that.

Take a look at the data in Charles Murray's book Human Accomplishment. Even though you have to ignore his Politically Correct milquetoast conclusions, the data he presents are irrefutable. It would be the ultimate sin against all humanity, against evolution, against Life itself, if we allow its most creative strains to be bred out and go extinct.