The main source of subway riders is feeder routes, not local density.
SSE will have decent ridership, with or without rezoning.
Sheppard subway extension would have less ridership than SSE, even with extensive rezoning around every station. There isn't as many riders wishing to go towards North York as towards downtown.
I agree that areas around subway stations should be rezoned for high density, whenever a subway extension is happening. High density is an added benefit in that case. I don't agree with density being the main purpose of subway extension; you just won't get enough walk-in riders. The total population and the transit usage in the area served by the feeder routes are the keys.