That rule will never be enforced by the USSF unless a bunch of American teams fold. The federation would never stand in the way of a Canadian team entering even if it put the number of teams above 25 percent. The USSF isn't as pinheaded as the CSA.
This. Folks need to understand that the USSF is trying to create something stable and viable. They're not looking for some excuse to suddenly pull sanctioning.
I think with the recent report on the viability of a Division 2 league in Canada the 75% rule might need to be changed? Or else teams starting in Canada will either have MLS or the new Division 3 as their options and nothing in between. I'd imagine Hamilton (who've shown interest) Calgary, Winnipeg, Quebec City and even a team in Toronto might be viable at D2 level. It's sad to see the Islanders go but those island flights might help with the travel budgets a small bit?
Those cities could be viable but I wouldn't expect it to happen so quickly that US club development couldn't keep up. The current round of expansion has three US clubs coming on board around the time of Ottawa and providing the needed balance. That pattern of growth could certainly happen for the long term. A Canadian D2 league that would be viable on its own would need to secede from the existing structure and I think they're a bit of a way's off from even contemplating how it would be done in practical terms. They'd need know what specific clubs could be in the game before they could envision how a stand-alone league might work. A lot of teams need to be created by bunch of unknown someones before that can happen.
I'm also pretty sure USSF, the NASL, and USL would want to wait until any/all match fixing probes are completed.
The CSA report has determined D2 isn't viable as a standalone in Canada and with Edmonton already in NASL and Ottawa coming what would be the point in forcing them out of that league into a Canadian one. I think the report is right in allowing MLS and NASL to cover both countries with existing teams already part of that structure. You're dead right that US development should keep up. There's a much broader base to expand to in Canada but should owners suddenly appear in the next year for say Hamilton and Winnipeg AND the Islanders come back next spring. Then there'd need to be 6 more US based clubs under the 75% rule. I don't think the USSF would keep those 2 teams out if it meant the league was growing as Bluesfan stated above.