The point of the Vaughan subway was to connect to York University, mostly; Vaughan got provincial funding for political reasons. I think that extension should have stopped at york U, the rest is unnecessary.
I'm not entirely against Scarborough getting a subway extension, but It seems incredibly wasteful to entirely mothball the RT, which could be extended and serve so many more people than a one stop subway would.
It's not really a cost thing in my mind however, I just see the potential to serve a much larger area with an RT extension than a one stop subway, which would actually serve fewer than the RT does now.
I would be in favour of upgrading the SRT, extending the platforms, replacing the trains and extending the line to Malvern via Centennial, with a better configured multi-modal hub at Kennedy that makes it easier to transfer to GO RER from the RT, which I think would be better for Scarborough residents heading downtown.
The subway is just going to continue to get more crowded, and the ride from Kennedy to downtown by subway is really long anyway. Based on distance to downtown, most people in scarborough aren't bound for the city centre anyways, and so they would be better served by a longer line with more stations.