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There are a few things wrong with this argument:
1) New tracks would be built in the hydro corridor, through back lots which currently only have parking lots, and in undeveloped areas near the 404. Prime land perhaps, but not developable land.
2) Having worked there myself, Beaver Creek is at least 15-20 minutes away from the nearest GO station, by existing public transit. This is due to chronic surface congestion in rush hour. I would agree though that in the off peak and because no one is working, 5-7 minutes is realistic by 407 only.
3) You're assuming that people in Thornhill and Richmond Hill actually use the GO line, which they don't in large numbers. The small (by GO standards) parking lot at Langstaff doesn't even fill up - unheard of lack of demand for GO. Of the 8000 daily riders that use GO, maybe 5000 originate in York Region. Those who live in Thornhill could just as easily use a new station at Thornhill Square.
4) Due to the nature of travel in the Yonge corridor, the subway makes sense - as evidenced by the concentration of bus routes that serve Finch, and the much larger parking lot that fills up by 7:30 am. If GO made sense, more people would actually use it considering that it is currently faster than taking the bus to Finch and riding the subway. Might as well end a redundancy of service, and finally provide rapid transit to York's largest office park.
The way that things are today do not necessarily have to stay as such indefinitely. When I said under 10 minutes distance I was referring to travel via the VIVA BRT which only stops a handful of times between the 404 and RHC/Langstaff GO. Once upgrades are made to the corridor such as true transit signal priority and queue-jumping the average travel times can be reduced sharply. You're right that to Unionville would be slightly longer but that is not the major travel pattern anyway.
Ditto the Richmond Hill GO line as is can be improved. If the train only ran during rush hour, every 30 minutes and cost a lot to ride I'd find it unappealing to use as well. With electrification of the Bala Subdivision and removal of freight, transit trains would have complete right-of-way, be capability of accelerating and braking much more efficiency and therefore can operate faster, achieving top speeds not dreamed of along the Yonge Subway, even on its north-of-Eglinton stretch. The journey would be very comparable to the RHC extension in terms of travel times from Langstaff Gateway to Union Stn. While the subway would take one 45 minutes to get from start to finish, the existing line electrified could take less than thirty. With such time savings, even a minor backtrack on the subway thereafter (be it at Oriole/Leslie, Wynford Hts or Union) would be marginal and most customers would not consider it an inconvenience.
Other North American cities with metro lines into its suburbs (New York, Chicago) other commuter services quite similar to my suggestion here. The Purple Line of Chicago for instance runs from the satelite city of Evanston, IL into the downtown loop. However this line runs non-stop express bypassing all the stops through Chicago until it reaches the suburbs at Howard Stn and stops thereafter are locally spaced. Through that non-service section there's a parallel metro (Brown Line) that serves all the local stops prior to Evanston but it does not leave the metro area. At Howard, people on-board the Purple Line have the option to switch onto the Brown Line if they so desire one of the intermediate station stops or they can remain on the train bound for the CBD. Liken this to an interlined Sheppard car meeting commuters at Oriole/Leslie then serving all the stops between Sheppard and Union.
Under such conditions the justification for expanding Yonge northwards dissipates because the major commuter line is now to the east, closer to Beaver Creek and Markham where the majority of trips originate today. Yonge/Finch only sees a high-level of commuter traffic because surface rapid transit as yet is not very competitive against metro subways. This however can change. Were there a similar transit terminal connecting to N-S mass transit at Seneca College; demand levels to Yonge St would not be as high. And RHC can still emerge as a major destination, it shouldn't be contingent on whether a subway actually gets built there. Should the DRL be built however and extended to Eglinton East it could also intercept the Richmond Hill REX line at Wynford. As such, customers on-board the REX commuter railway would now have direct and relatively direct access to all major crosstown subway and LRT lines.
Node-wise, you'd be interconnecting far more priority areas with mass transit per not extending the subway and using that expenditure to build several projects instead. One such project, surface transit down Yonge on dedicated ROW, would not have the large kilometres apart spacing gaps of a subway as what's planned for Thornhill. You could have stops at Steeles, Doncaster, Clark, John, Royal Oak, Helen, Langstaff, RHC, High Tech, Bantry, 16th, Weldrick, Harding, Major Mac; thereby improving the quality of life and local accessibility for pedestrians as well fostering a genuine streetlife and streetscaping through all of urban Richmond Hill. Relocating the point whereby commuters switch from their cars or the bus onto the subway a few kilometres up the road is not a real solution. Getting commuters to convert completely onto other services by making them competitive against the Yonge Line for inbound/outbound travel is, and that like RHC extension will take money to do.