OK, let's say Canada annexed those states. We now have the United Socialist States of Canada and "Jesusland". All the Lefties flock to Canuckistan and all the Conservatives move to "fly-over country". Canada gets its socialism, healthcare for all (which it already has and is shit), Hollywood, some tech companies, all the cities with the highest crime rates, all the illegal immigrants they want, and all the people who are happy to live their lives on welfare. What's left of the US gets to finally secure the borders, gets to streamline government, do away with social programs, get rid of the restrictions on drilling for oil/digging for coal/building nuclear powerplants/manufacturing/etc., and finally get rid of draconian gun control laws.
Big companies will choose "Jesusland" because they won't be taxed into oblivion to pay for social programs and can actually afford to manufacture things again. Unemployment and crime rates go down. Everything becomes more affordable. The lower class will virtually be eliminated. Everyone has more money. The US becomes energy independent. Everyone speaks the same language. Criminals and drugs are no longer flowing over the border. We still have our military, Disneyworld, Las Vegas, and all the best national parks. Anyone can own whatever guns they want (I really want an M240LW and one of those Knight's Armament belt-feds). We still have all the food production, with less mouths to feed.
Meanwhile Canada will be struggling to feed everyone, their taxes will skyrocket, their unemployment and crime rates will increase exponentially, and they'll have more and more people flooding their borders from around the world to get free shit. They won't be manufacturing anything, exporting anything, and will struggle to keep their lights on, houses warm, and people fed. They will mostly be reliant on imports for food, oil, materials, and goods. They'll allow anyone and everyone to come there, because they don't want to be racist, so they'll be dealing with illegal drugs, disease, and crime. Within a decade becomes a Third World country.
You know what? Let's do it! I'm 100% on board with this plan. Give me my machine guns, Bible, low taxes, and cheap oil, and the pinko Lefties can have their multicultural, economically equal, environmentally friendly, all-inclusive Socialist Utopia.