While it's true that a 31 mile (50km) subway line is a little absurd for Toronto, the fact still remains that New York has a subway system that is 170% the size of Toronto's when compared with population (in other words, per unit of population, the NYC subway is 70% larger than Toronto's, and of course, about 1/3 of their system is aboveground, which is something Toronto should really look at). It should also be noted that Toronto is still growing. While the existing enhancements (Crosstown (underground), SSE, DRL) will close that gap significantly, you would have to also build the Yonge North subway extension, RLN, and RLW in order to be fully on par with New York in terms of subway length/population. It should also be noted that New York's subway trains are significantly larger than ours, so total capacity is actually much larger in New York than it is here. Finally, when you consider the fact that "route miles" does not include the presence of express lines, the length of the New York city subway increases by about 50%. If we are ever to catch up in terms of capacity here, we would need to build a full Sheppard line, subways further into Mississauga, another relief line on Dundas, Relief Line Northwest, and Quad track the Yonge line.
Also, almost no one in the suburbs of New York City (Bronx, Brooklyn, Queens) uses the LIRR or MNRR to get to and from work -- everyone takes the subway. Having the subway extend to the middle of Scarborough and Etobicoke (in the long term anyways) is not far fetched, especially for a growing city of our size.
A better city to compare transit to is Chicago, which 2.25* the size of our system in population/km metrics. While the vast majority of their system is aboveground, it shows what rapid transit can look like in a city relative to our size.