Not sure about the stations located where they are only because of political reasons. These areas near Rosedale, Summer Hill and Old Mill might not have been rich neighbourhoods at the time when the stations were built. The proximity to the stations definitely drove property values up in the area, but I don't think the stations were built just for that reason.The least used stations are the ones that are there for political reasons, or because people expected the subway to drive development.
Nobody uses rosedale, summer hill, or old mill but they exist because rich neighbourhoods nearby wanted to bump up property values by saying they were near a subway.
Glencairn never attracted the promised development. Stations like Chester have also never really spawned a lot of development despite being there forever.
The idea that a subway will double the population of Scarborough is silly.
Put a subway into an area you are projecting will grow on its own anyway but don't expect the subway itself to drive growth.
You are right in that a subway won't magically increase population density. I'm going from memory, but the area of the proposed Scarborough subway is low density residential and commercial. For the subway to have any impact on population density in that area, you'd have to bulldoze a lot of single family homes and businesses and replace them with high-rises, which will never happen.
I imagine that zoning restrictions and community activism determines how much high density development goes on in a neighbourhood. If people don't want high-rises in their neighbourhood then developers simply won't go there even if there is a subway in the area.