While this is true it is interesting that the original tribal ancestry is never lost. The descendants of Judah, Levi, and Benjamin knew who they were (at the time of Christ, and maybe later as well.) This is important as it relates to the end time prophecies concerning all the tribes of Israel. While the northern kingdom is now 'lost' it must be 'found' to fulfill these prophecies. Many, including myself, believe that the WASP's comprise most, if not all, of those lost tribes.
I am familiar with this theory, but I don't really agree personally. I have also read that the natives of North Central and South America comprise a good portion of these Lost Tribes. This seems to be supported to some extant by the writings of Esdras (Ezra) in the apocryphal 2 Esdras chapter 13.
39: And whereas thou sawest that he gathered another peaceable multitude unto him;
40: Those are the ten tribes, which were carried away prisoners out of their own land in the time of Osea the king, whom Salmanasar the king of Assyria led away captive, and he carried them over the waters, and so came they into another land.
41: But they took this counsel among themselves, that they would leave the multitude of the heathen, and go forth into a further country, where never mankind dwelt,
42: That they might there keep their statutes, which they never kept in their own land.
43: And they entered into Euphrates by the narrow places of the river.
44: For the most High then shewed signs for them, and held still the flood, till they were passed over.
45: For through that country there was a great way to go, namely, of a year and a half: and the same region is called Arsareth.
Interestingly enough, research will show that this scripture played a vital role in Columbus' belief that there was land "out there." He also took with him Hebrew interpreters to aid him in communicating with the people he was sure he would encounter.