WHBM,
I don't think Montreal's loss of aviation pre-eminence in eastern Canada is directly related to Mirabel's opening, tempting as it might be to see a causal link. Its heyday had, I believe, more to do with the fact that it was the financial capital of Canada, a position it has since ceded to Toronto. I also think the ICAO headquarters may have had something to do with so many intercontinental flights choosing Montreal over Toronto.
Montreal never needed another airport, unlike London. Powerful city notables had big dreams (Expo, the Olympic games, a "CDG-style" airport). These were seen as expressions of "national" ambition (by "nation" I mean Quebec not Canada!). In Europe, Barcelona is similar, only more successful. Sending intercontinental traffic to Mirabel only exacerbated existing problems by discouraging connecting traffic.
Returning to London, I think that the days of its pre-eminence might be numbered, not least because of transportation problems (not just its airports, but also the tube, roads and trains). I wouldn't be surprised if there's a drift away to European cities which seem more attractive. Of course, I could be wrong. Just look at New York - similar transportation problems, high cost of living etc. yet still very much the "world city". Perhaps that's it: world cities prosper despite themselves!