I don't think we will need to worry about that. Off the top of my head, any land developer would look at these areas first within the 416: Humber Bay, The Queensway, Six-Points + Dundas West, Unilever, East Bayfront, Lower Don Lands, Portlands, Eastern Ave, Yorkdale, Eglinton-Don Mills, Lawrence-Don Mills, any point along Sheppard, any point along the 401, the Golden Mile strip, Newtonbrook, Steeles Corners.
All these areas are more desirable than STC, subway or no subway, and would cost the same to build something in as STC. This isn't even getting to hub spots planned and underway in Markham, Richmond Hill, Vaughan, Mississauga.
I see zero reason why a subway would suddenly make building at STC preferable to developers when they can make attract more buyers and make more money at any of the above locations at the same price.