Have you driven down Hurontario lately? The section from Eglinton to Cooksville GO is almost like driving on Yonge St between Sheppard and Finch. It's the densest corridor and area outside of the City of Toronto in the entire GTA!
And I find the argument of "well there isn't much along Dundas, so there's no sense in building a subway there" to be quite dubious. Yes, I agree, on it's own there isn't much to warrant a subway, but none of the major E-W corridors between the 427 and Hurontario have much on them to warrant a subway. But the fact of the matter is that Hurontario needs one, so might as well pick a corridor that has good opportunities for densification to get it there. Remember, there isn't much along the Scarborough RT route between Kennedy and STC, yet the line is still very well used.