Here I disagree. That's not the "real reason." The real reason - which many still find difficult to accept - is to enable massive suburban intensification in a key node, the plans for which go back concretely more than 15 years now and, really, another 10 or 15 years prior to that. I don't really care whether Toronto residents are frustrated because they don't understand the Growth Plan for the Greater Golden Horseshoe. I'm not going to roll my eyes for the umpteenth time when I hear David Miller opining that RH residents really would be best served by the (not viable) GO improvements when he and I both know that he probaly hasn't been within 10km of Silver City Richmond in his life, much less reviewed the (pre-MZO) Langstaff Gateway Secondary Plan. Objectively, that position is simply wrong and (speaking to what I've heard him say, not you) is ignorant of intensification patterns and plans as well as how people in south York Region travel and use transit.
As for Vaughan "already getting a subway," it's 2.5 stops on the opposite side of the municpality and, again, Torontonians can be frustrated if they're ignorant of growth in the 905 but I'd suggest they should be far more concerned about their own declining inner suburban population, how they've messed up transit in Scarborough (which will become VERY clear before the end of the year), how they're wasting money on SmartTrack and the Gardiner, and a half dozen other transit/growth-related issues before dwelling unduly on how "their" infrastructure is allowing for less auto-centric development in their suburban neighbours. IMHO.
Agreed. This, no one can dispute.