For most people the city does not end just at the Toronto borders, we are one region. Companies need people from every section of the GTA to find people, and getting people to and from work around the GTA should be pushed daily. That means the TTC should expand to where service can easily be provided. Extending Yonge line North to Richmond hill, what 3-4 KM out of Toronto proper is hardly a far flung place. It is a part of the region where every lot is being turned into condo's. There is a reason Finch station is packed, people from all over bus in. Realistically that money won't come from Toronto to build it, it will come from the province, york region and the feds.
Look at the mess around the airport, its the 2nd largest job concentration in this country. And public transportation is a mess in that area, so everyone drives. The issue there is that region is split over 4 cities, and when you are outside of Toronto by 2 KM (despite being the 2nd largest job zone in the country) the TTC is horrible. So if you are at the Bell office 1KM from the Toronto border you are adding in a long commute, so most people will just drive. That same issue happens in Vaaughn, some drive, some Go train if they are downtown Toronto, some bus to Finch and TTC, but it really isn't clean for anyone.
Experts want it build, demand says it should be build, growing part of the region. Not sure why people get all protectionist, its not like the Toronto budget will pay for it.