"Not everybody is going downtown!"
I'd say a majority are headed for the subway at Kennedy. Even for those that aren't going downtown, the subway would help many of them get around Scarborough quicker. Even if they built umpteen more GO lines and LRT lines, STC to Yonge & Bloor by a direct subway line would still be a very competitive 25-30 minutes.
"The region will need a heavy backbone at some point and I don't see any advantages in deferring the subway into the future."
Exactly, as if the LRT option is really cheaper, anyway - half a billion seems to exclude an extension to Malvern and any links to the existing streetcar network downtown (since LRT here seems to mean nothing else but "streetcar in ROW"). Extend both B/D and Sheppard to STC and that's all the subways Scarborough needs for a very long time. I've said before that ridership would easily exceed 70,000 immediately since the corridor is at 60,000 rides daily right now when you include the 6 million lost rides per year. Is the Vaughan line forecasted to have that many in 50 years even? Councillor Thompson said 26,000 more people will live in the area in the next little while, and I bet that doesn't count the people that will move into developments triggered by the subway - and STC has tons of room for condos. The money and the "ridership" is there for the sinkhole extension - there's no reason why it can't be here for this extension.
"While not presented as an option, an LRT doesn't need to be on the same line as the current SRT. You could build it at grade along any path you choose, really. The City didn't present that as an option, but it could be one."
Are any streets in the area wide enough?