I mentioned something similar a few weeks ago. I've heard that the City, at one point, intentionally restricted development at Kennedy Station, so it wouldn't compete with Scarborough Centre (this might still be policy today) for development.
I feel like developers would be more inclined to build at Kennedy Station, with Crosstown East + West, Line 2 and RER converging there. Much like North York Yonge Street and North York Centre Station, Eglinton Avenue could be a medium density avenue, shifting to higher densities at Kennedy.
Anyways, good word is that developers have continued to show no interest in Scarborough Centre, even if the subway were to be built there.