One subway line running at 1/4 of capacity is not enough for a corner of the city. It clearly needs two so that it is equal in status to the core.
Here's the flaw with that argument: assuming that an extension to an existing line has to fill up before it reaches the existing line.
No normally operating subway ever operates at filled capacities in the outskirts of a system, ever. It picks up passengers as it goes along its route, only crush loading as it reaches the core. If you fill a train all the way up in one area, then it can't properly serve the rest of the line.
Even if the subway is 1/4 full by the time it reaches Kennedy (It'll almost certainly be more than that), that's still between 300 and 400 passengers per train every 2.5 minutes, or around 8.5K PPHPD. Compound that with an existing 20K PPHPD on the rest of the Danforth subway, and your train is crush loaded when it reaches Bloor Yonge — Exactly how things should be.
For context, look at all the other outskirts of the subway system during the AM peak:
Etobicoke (Line 2 West Side) — ~130K PPD. Trains have open seats till around Runnymede. Crush Load occurs at around Ossington.
Downsview (Line 1 Sheppard West South, prior to TYSSE) — 110K PPD. Trains have open seats till around Yorkdale. Crush Load occurs at around Spadina
Vaughan (LIne 1 TYSSE) — 90K PPD. Trains have open seats till around Wilson, with reverse-commute trains being standing room only until York U. Crush Loading occurs at around St. Clair West
Sheppard (Line 4) — 50K PPD (shorter length though). Trains are standing room (occasionally crush loaded) only past Don Mills, but run smaller trains and less frequently.
North York Centre (Line 1 Yonge Side) — >200K PPD. Trains do not have open seats past Finch, and are crush loaded south of Sheppard.
Scarborough (Line 2 West of Kennedy) — 150K PPD. Trains do not have open seats after Warden. Trains crush loaded at around Pape, then again at Castle Frank.
The SSE is going to get at least 100K PPD (SSE and Sheppard east will be very much like Sheppard West station prior to the TYSSE, which saw around 40K PPD. Lawrence East will be like Lawrence West, and will likely get around 20K PPD), likely closer to 120-130K PPD once ridership matures, which is only 1/3 of what a subway is capable of. However, it's very in-line with the ridership of all the other extents of the system.
And if the argument is to be made that then Yonge subway fills up early, just remember that we're planning a brand new subway line to reduce that ridership. Ridership levels that high are not sustainable.