Flint does, however, explicitly bring up the root issue behind "No Way To Contain:" there were a LOT of Anglos coming in, and coming in constantly. By the time of his point of departure (1814 or so), the demographics were just unsupportable for Native Americans. Ethnic groups whose populations are measured in the tens of thousands at most aren't in a good position to meaningfully draw a line on the ground and saying "thus far, no farther" to a population of millions that has no ethical qualms whatsoever about taking their stuff. They can try, but of the available options, none end well.
Honestly if you want to write a storyline where European settler-colonization of North America never happens, you probably need a point of departure prior to Columbus, and a fairly drastic one.