Montréal in the 19060s was the undisputed centre of Canada for business, culture, population, sport, politics, and international relations. Canada expected to grow by millions of people, and we expected a large number of them to settle in Montréal. 35 miles was a modest buffer zone anticipating enormous suburban sprawl.
The whole project was derailed in the 70s by the terrorism of the FLQ, and then the 1980 secession referendum. Suddenly, banks, insurance companies, and foreign offices took flight and moved on down the road to Toronto. Montréal _lost_ hundreds of thousands of high-paying jobs, and hundreds of thousands people as anglos and jews fled. Montréal sunk into a 15 year depression. Toronto became the new centre of Canadian business, culture, and sport. Toronto became a major Jewish city, and immigration shifted for good. The influx triggered a property bubble (and then the condo-crash of 1989).
The growth of Toronto of the last 40 years was planned for Montréal, and had it happened there, Mirabel would be a thriving international airpot. Toronto was unprepared, but Montréal had been planning for decades. To this day, Montréal _still_ has more subway track than TO.