Looking at the map.
I just realized... SmartTrack alignment is aligned in such a way that makes Airport Corporate only two SmartTrack stops away from a theoretical Missisagua Square One stop!
Airport Corporate Centre is only as far away from Mississauga Square One, as Airport Corporate Centre is from when it spurs out of the Kingston line (just before Weston). So that's a relatively short extension. That brings a theoretical Square One tunnel for far less price than the Scarborough subway extension!
They could bend it slightly south (after Airport Corporate or a subsequent stop such as Dixie/Tomken -- another obvious first stop for a SmartTrack extension) into the north edge of downtown, or even the existing GO bus station there. That makes SmartTrack even more brilliant (assuming province forces the use of longer GO RER trains instead of tiny UPX trains, as a condition of funding Tory). SmartTrack would continue onwards and interchange GO Milton Erindale(existing) or GO Milton Eglington(new). If SmartTrack reaches both Mississauga downtown (only needs doubling of the Eglington spur length), and the extension to interchange with GO Milton (only needs a tripling of the Eglington spur length).
Downtown Missisauga/Square One gets a subway via a small SmartTrack extension costing far less than the Scarborough subway, even if it has to be underground all the way from Airport Corporate! (15-min allday, 5-min peak underground GO RER trains that are branded as "SmartTrack" or other brandname)
So, within less than one generation -- we can catch a "subway" from Mississauga downtown to Scarborough downtown, if living in one and working in the other. Faster than by car.
Tory, brilliant, Tory. (whether you like him or not). He's thinking more ahead than I thought.