You can't base the argument on modern social norms. Although you could possibly be correct, your argument can't ever get beyond assertion. And there are plenty of exceptions to the rules you mentioned, so cultural exclusivity will always be full of holes.
But the key question remains, even if there was interbreeding, how could one entire line go extinct when it stretched across the Eurasian landmass?
That's ridiculous. I am not basing the argument on modern social norms. I am basing the argument on universal human behavior. You can go talk to the most 'primitive' humans on earth if you want proof. Opps!! Wait, you can't. Those would be the Sentinel Islanders and they're so xenophobic they won't let anyone even land on their island and no outsider has ever talked to them.