^ The subway won't be finished for a decade if started today. The corridor most definitely has the ridership for a subway - aren't 6 million trips per year are lost because of the poor service the current RT provides? I sure believe that number - I avoid the RT on my way home now and only take it in the morning. Add current ridership (42,000 boardings per day in 2004), lost ridership (~16,000 boardings per day), ridership on competing alternate/diverting routes that would switch to the subway (over 10,000 for sure), and 10 years of growth (they're still building on greenfields in NE Scarborough) and you're easily looking at over 70,000 trips a day.
There may be places in the city where a similar length of new subway track would be "better" and busier, but extending the B/D line to STC is punch-you-in-the-face obvious. Then run Sheppard over to STC and we're done for now and can build new transit lines elsewhere (don't extend either to Malvern). During subway construction, a busway or express buses can easily replace the RT - if they really want to be nice to Malvern commuters, they'll run some GO trains along the Midtown route to Summerhill station.