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Good Point. The entirety of the North American Indian legal status rests on their claim to have been created here. That claim gives them on-reservation Federal tax exemption, the right to own and build nontaxable casinos, fireworks and tobacco sales, as well as unregulated natural resource access - whatever the tribe wants, they get.
That claim also gives them Constitutional Rights, while relegating all others rights to the status of privileges which, according to SCOTUS, can be revoked at any given time.
Break that claim and the whole Treaty tribe empire falls.
Their legal status rests on the treaties they have with the Federal Government. It does not matter if they have been here 10 million years or 10 minutes.
In fact several of the treaty's refer to lands that the tribe had very recently moved into.
The Dakota area is considered Sioux country but it had been owned just a little bit before by the Cheyenne who had taken it, I believe, from the Kiowa. Doesn't matter, Lakota reservations are in The Dakotas, Cheyenne are in Montana and the Kiowa are in Oklahoma.
So your little fantasy is not based in reality. Unless the US Federal Government wants to abrogate all their treaties with all the dependent nations things stay as they are no matter who was on first.