The Buffalo Bills were playing regularly in Toronto for a few years in the late 2000s when the franchise was in the worst of their doldrums and the city was a lot worse off than it is today. It sure seemed like the plan was to gradually migrate games to the bigger market. Then they got competent again, Pegula bought the team, and the NFL became so flush with revenue that market size stopped mattering.
Market does matter, in the NHL at least. I wouldn't be opposed to the Sabres looking at some kind of arrangement with Hamilton if it means that it'll put the organization in a better place. Bettman isn't going to let the team move from that region since it's still hockey-crazy, but a two-city solution would be interesting. There's plenty of other markets where that may work as well. The Hurricanes could tap into Charlotte more, even though Raleigh is doing fine and they have a good fanbase and competent organization.