PaulG, you sound like a refugee from the 1960s, when blockbusting was very much in style. Although I don't think the city expropriated anything, in those days. Seriously, your suggestion is contrary to any currently accepted planning principles. These days we emphasize maintaining the stability of neighbourhoods (at least in theory). Ripping apart established neighbourhoods is a non-starter.
Most of the B-D subway runs through stable residential neighbourhoods, dominated by detached and semi-detached houses. There is probably room to intensify directly along the main streets (Bloor and Danforth), up to perhaps five or six storeys, but on the side streets most people would say, we like it as it is, thanks.