If you put it that way, I read it as "in contrast with e.g. Germany or France". However those countries are also built on citizenship. Oh of course you will have far-right people wanting to stop all immigration (and be racist against people they don't find similar enough, physically, culturally, or because of some arbitrary amount of ancestry origin), etc. but given that the same thing happens in the US, I don't get the substance of the difference.
You aren't a nation until you become one. And I would find it depressing to refuse to become one just because of the variety of random physical traits and/or a kind of fossilization of cultures. Complete assimilation and convergence to a mean may be undesirable, but even more is excessive segregation...
So the question can boil down to when can the "common ancestral origin" stop? Because if e.g. 3 generations is enough (and why not) it could be easy enough to agree on becoming one.