What do you mean when you say "urbanized"? I'd say it's more like 40% of the 905 will hopefully not get urbanized, maybe 60% not being urbanized at the best. But you aren't talking about that kind of urbanization are you?
I think you'd be surprised at just what can be done with the 905. I know in Markham and Durham, the new subdivisions are getting tighter and tighter, getting closer to resembling the denser neighborhoods close to downtown, even more than places like Bloor West or the Danforth. There's plenty to go on in those areas, in terms of both transit and urban development. It might take some buildings to be knocked down, but it's definitely doable. Suburban arterials can easily be changed to add in BRTs down the middle or 4 storey mixed use buildings on either side, and they could be built to blend right into the suburban landscape. Something like Kennedy Road from 16th to Major Mackenzie has a huge number of options to make it more walkable and more transit-friendly. Same with Brock Street through Whitby, or Steeles Avenue. And at that point, major arterials like Highway 7 and Hurontario will have enough pressure to demand LRT or even subway, on top of the very dense building that's already slated to go up all along those streets.