You cannot revoke a person’s US citizenship simply because the territory where they live is no longer part of the United States.
The best example are US citizens born in the Panama Canal Zone.
When the Canal Zone was turned over the Panama, the US citizens living in the Canal Zone Did Not Lose their US citizenship.
Furthermore, the children born to US citizens born in the Panama Canal Zone are also US citizens and their children, the grandchildren, great-grandchildren and so on will be US citizens.
That is why John McCain who was born in the Panama Canal Zone , is still a US citizen.
I conceded the point on revoking citizenship several posts ago. My point is that FUTURE generations would not be citizens just as those born in the Panama Canal zone which is no longer US territory are not citizens. Should Puerto Rico become independent then they would be Puerto Rican citizens.
You are leaving out the residency requirements. For example, a child born outside the USA (such as in the former Canal Zone) to a US citizen mother is not a citizen unless the mother resided in the USA for a certain amount of time prior to the birth. There are different requirements depending upon whether citizenship is derived from the mother, the father or both parents. Over time, as a territory is no longer part of the USA, the chances are high that even US citizen parents will not meet these requirements such that their children will not be citizens.