As far as the US goes, soccer is already watched by nearly twice as many people as their preferred sport than hockey. The problem is that viewership is split between EPL, other Euro leagues, the Mexican league, and MLS.
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Which is why the CPL has potential locally. If it can even come close to the local loyalty that is found in Scandanavian countries, the CPL will be fine.
As for MLS in Canada:
The 3 MLS teams would likely stay for the economies driven by US soccer.
TV viewership in Canada involving Canadian MLS teams is higher then TV viewership for US based teams in the US. (see the MLS Cup final - 1.3 million up here - 800K in the US) MLS knows Canada is a HUGE potential lucrative market that will drive local teams that, given the right ownership, can spend.
MLS also recognizes that unlike other sports, a Canadian soccer team has serious potential to garner local support across the border. IF TFC were to go on a 5 year run of championships & success, the team would find followers in the US.
[Murphy Malone is making believers down there one tweet at a time :) ]
All power to the CPL - may it drive soccer interest in local environments and make this game the top sport in this country eventually. BUT, the North American league is in Canada to stay.