Had the US annexed Quebec two centuries ago, the massive American population influx into the region would no doubt have assimilated the Quebecois, and today French Quebec would be a bunch of place names around the state, and tourist districts in Montreal and Quebec City. That wouldn't happen today because there is a substantial French-speaking population, and a new large English-speaking influx is unlikely.
And yes, I believe English is fundamental to being part of American culture. That doesn't preclude speaking another language as well. But the descendants of hispanos of New Mexico, Hawaiians in Hawaii, and every other group speak English, and would be the first to tell you of the importance of doing so for personal and societal advancement.