If America vanished, something must fill the void, and probably China whose freedom of navigation, trade, and political forces would not be kind to Canada in this hypothetical. Also, whatever "ruled" the space that was the U.S. would probably be more belligerent without the relatively superpower in the neighborhood.
Say Brazil gets more authoritarian leadership, as tends to happen from time to time: "Why not openly loot Bolivia? Did you see what happened to that other country... war is not economically viable either with all this stability..." suddenly, you remove that stability, you remove those military and economic consequences, you remove the guideposts, the fences, and the archon of freedom and liberalization... who will run the neighborhood now? Uh oh...
We are all deeply influenced by power and culture, and there is no escape, just a chance to choose how to live by them.