High Park was along the 2nd busiest streetcar line in the city, which the Bloor-Danforth replaced. There was already an urban grid in place, which Bloor West Village very close by.
Old Mill is between Jane and Royal York stations, both of which have very high traffic.
It will take far more than the development of the Canadian Tire lands to justify a subway there. Bessarion, according to 2016 stats, has just 27% of the riders that High Park does - and High Park has Keele station (14,610 riders) less than 600m away, and Runnymede (nearly 20,000 riders) less than 800m to the west.
That short stretch of three stations handles nearly 45,000 riders a day.
Bayview, Bessarion and Leslie handle less than Runnymede alone.
This is exactly why the Sheppard Subway was a huge mistake. There's no need for a subway there. Spending money on subways in areas not suitable for subways is idiotic when there are many other areas of critical need in the city. Even after 16 years, the ridership and development in the area is nowhere near close enough to justify it.
An LRT would've been fine on Sheppard, and still will be for the rest of it. The Sheppard line could've been postponed for about 80 years, and considered at a time when it actually made sense.
If I recall correctly, the Sheppard Bus was once the busiest bus route in all of Canada...It makes sense now with all the development that is ensuing along the line. As I recall, the section you mention has been around for about 50 years. That's not a fair comparison given the amount of time the BD line has had to develop.
If you change your catchment area to Sheppard Yonge, Bayview, and Bessarion or even Bessarion, Leslie, Don Mills, the ridership is about the same. You can pick any section of the system and you can find low ridership.
- Try The Spadina Subway between Museum and St Clair West, Spadina and Dupont only get about 10-15K passengers each, and Museum only about 8K, quite low for stations so close to downtown. (St George is the exception because it's a transfer station).
- What about between Bloor and St Clair? Summerhill and Rosedale see only around 6K PPD, meaning the 2 km section between St Clair West and Bloor Yonge only sees 6K passengers per km, lower than that of the Sheppard Subway.
- The Donlands, Greenwood, Coxwell, Woodbine section of the subway is almost 4 km and only sees about 50K passengers per day, similar to that of the Sheppard subway.
- The subway between Yorkdale and Dupont (Not including Yorkdale or Dupont) sees about 50K passengers per day with a distance of 6 km. The Passenger/km ratio there is even lower than the sheppard subway.
The point is that you can't judge the stub for low ridership when comparable areas all over the system have ridership similar to that of the stub. No one complains about Summerhill/Rosedale, or the subway through Allen Road or through the annex because the few stations that get huge ridership (Finch, Sheppard Yonge, Eglinton, St Clair) make up for everything.