Andrew, residential developments along corridors don't contribute very much to rapid transit ridership in the suburbs. You could built multiple
Mirvish + Ghery's along Sheppard and it still wouldn't have enough people living along it to justify a subway. If you want to use new developments as a justification for a subway, they'll have have to be commercial/office space. Of course, no matter how you look at it, there aren't enough condos or offices along Sheppard to justify a subway.
Now, not all hope is lost for a Sheppard Subway. Suburban subway projects are justified when they have enough ridership on bus routes to feed the subway. This is why Downsview Station and the Spadina Subway is successful. But unfortunately for Sheppard the bus routes don't come close to being able to justify a subway.