I'm not sure how accurate that map is, but it begs the question why didn't just build the underground portion as subway and have a subway station at Consumers? I know a one-stop extension it probably isn't worth it for. I guess they'll just convert it when they build the rest of the Sheppard line. What that map does show is how "local" a service this line is. A subway from Don Mills to STC doesn't really duplicate much of the SELRT. A Sheppard Subway would have to turn south probably around Agincourt I'm guessing, which leaves Midland, Brimley, Brownspring, McCowan, White Haven, Shorting, Massie, Markham, Washburn, Burrows Hall, Neilson, Murison, Brenyon, Morningside, (Dean Park, Idagrove, Meadowvale Rd) which aren't being served by a Sheppard Subway anyway.
Let's look at the duplicated part:
Consumers, Victoria Park, Pharmacy, Palmdale, Warden, Bay Mills, Birchmount, Allanford, Kennedy, Agincourt.
Subway stations: Consumers, Vic Park and/or Pharmacy, Warden, Birchmount, Agincourt might make a good GO/LRT/Subway interchange, and then dip down to STC.