karmoka
Headquarters and capital were already moving towards Toronto by the 60s, following the opening of the St-Lawrence Seaway, the growing importance of the american economy for Canada and the progression of the quiet revolution which saw the province become way less interesting for the old money from the Golden Square Mile.
The FLQ was also most active in the 60s, culminating in 1970 with the October Crisis and then nearly seized to exist in the 70s/80s bar the occasional bombing by the RCMP (see keable, mcdonald commissions and Robert Samson)
The 1980 did lead to what a few economists called the "Montreal Effect", and there's certainly some factual basis to it in the form of the flight of many anglophones from the province, but arguing that the sovereignty movement was mostly if not solely responsible for this trend we were seeing before it was on the radar instead of precipitating the ongoing changes is overstating the case.
That said, Mirabel was a disaster regardless of the following growth of mtl. It was rushed for the Olympics, the plans to properly connect it fell before it became clear that mtl's population wouldn't grow as fast as predicted and airlines absolutely loathed it from day one.
However, it does make the point that it is akin to "L'affaire de Tarnac" that lasted from 2008 to 2018.