A couple tens of thousand is not "filled up", that´s basically nothing considering some of them would also emigrate. What kinda of guerilla resistance you have with that small recently settled population prone and easily incentivized to emigrate? Or possibly population that lived in Europe just some years ago. That´s why I think annexiation of the Western great lake region or Oregon country is possible.true, but they were settled from the southern end and there are a lot of people in Michigan even 1837 when it became a State (when it had over 80,000 people in it). American states east of the Rocky Mountains filled up fast when it became practical to settle them. Only the states in the Great Basin (Utah, Arizona, New Mexico, Nevada) or northern Rockies took a while to fill up. Even the Nebraska Territory had 28,000 people in it in 1860 and only the Central Plains and Rocky Mountain and Great Basin states were still mostly empty of non Native Americans