“Euro-American losses could thus be replaced, while the Indians, living in a subsistence economy with a low population replacement rate, could not.”
That’s it in a nutshell.
Despite occasional slaughter, the story of European settlers and native tribes was more coexistence to the point of intermarriage and genetic swamping than anyone getting killed off, truthfully. There’s a reason practically all old, native southern families have some legendary claim of indian ancestry. It’s because they do, way back, seventeenth century.
That and the Natives were still technologically in the Neolithic age. The didn’t work metal, weapons and tools were still knapped from stone. Other than dogs, no domesticated animals that I know of. There was no way that they were going to fend off the influx of Europeans.
It wasn't so much out breeding, it was out-immigrating. The Yankees replaced some of their losses during the War Between the States much the same way (while the Southern ports were blockaded).