That you see this as the one possible path, with an intercultural negotiation (as in, you know, Switzerland) being impossible shows me your estimate of 18th century Anglo-Anglo American politics overestimates the strength of its Hobbesianism and blood-and-soil nationalism and *significantly* underestimates its laissez-faireism. Forcible occupation and assimilation is the most impossible of all. How will a young, penniless, tired country, that gets massacred in battle by Native American warriors at St. Clair's defeat man, motivate, and pay for an army of occupation in the St. Lawrence valley to suppress the Franco-Catholic populace and protect Anglo-Protestant settlers?
Was being in such an army an appealing job for men of that generation post 1781 or so? Would the taxpayers of the 13 colonies gladly levy taxes to support such an army? Foreign loans, subsidies? from France, non, Britain, no, Spain, no.
There's some good farmland in the St. Lawrence Valley for growing grain if you don't mind shoveling alot of snow and cold, long, dark, winters, but how attractive is it for a mass volkwanderung of Anglos when you're moving next door to pissed off Frenchies who know the land better and seem to have a penchant for arson, when there are easier and warmer frontier lands to move into? Anglo-Americans had no public education system to really drill into them its their duty to take bigger risks to populate Canada in Generalplan Nord. Most individuals and families, even in New England and New York, and certainly more southern colonies, will take paths of lesser resistance.