Look at the numbers, Yonge from Eglinton to Wellesley is at or above capacity, particularly during the morning rush. It simply can not take many more passengers. Yet more people are going to want to be traveling that section when they extend the subway north and make it that much more convenient for people to get to with the construction of the Eglinton LRT (and other Transit City lines).
All those people in places like Thorncliffe you don't want to service with a new subway are still going to be taking buses to existing lines, lines like Yonge that are maxed out.
The primary point of the DRL, whether you agree with it or not, is to provide an alternate route downtown, bypassing Y&B, for the thousands of people from areas like Thorncliffe. This is demand that already exists, not potential demand to be created by development along the subway route.