By that logic, Torontonians shouldn't be allowed on GO and there shouldn't be GO stations outside downtown, any more than there should be a subway crossing Steeles. Are you offering this trade?
OR would you prefer to recognize the need for a hierarchy of transit services to deal with commuting population that is increasingly hetereogenous, destination-wise?
Interestingly, given the discussion of ridership at terminal stations, the Yonge extension should have very high ridership at its two final stations, likely not right at opening but within 10-15 years. I'll bet that Richmond Hill Centre station people pulls in more than Downsview does, to the extent that's relevant.
Even if, on opening day, the Yonge extension didn't have a single rider more than it does today, it would STILL be worth it. You have thousands of buses and thousands of cars travelling from Hwy 7 (and north) down to Finch every day. The benefits to the environment, the road itself, and to traffic might not be worth $3-billion to Joe Taxpayer but they're certainly worth a lot, IMHO.
Instead of cramming in everyone at Finch and having everyone drive down there you're spreading out existing ridership across several stations and moving the congregration point north (presumably to a new lot at Langstaff). All those people worried they won't get a subway seat at Finch should at least have a much easier time getting a parking spot there