Calomiris and Haber ("Fragile By Design") have run the natural experiment between the U.S. and Canada over the 2008 bubble period. The conclusion is the opposite of yours - Canada really just has the one bank, and sailed right through the crisis because they didn't hold as many rotten mortgages.

And in a way, the big banks are effectively nationalized in the U.S. We do maintain parallel governance structures because fully nationalized banks ( like the Bank of England in Britain's Mercantilist heyday ) are ... well, mercantilist.