I also want to outline why the Promenade detour is so god damn silly, and there should at least be a branch that stays on Highway 7:
1.Yes, you can look at it as a square with two sides going into the urban area of Thornhill and two sides bypassing it, but the thing is that the roads which bypass it are a continuation of the road it stays on. Simply put, it goes out of its way to reach the Promenade.
2. Besides the fact that the terminal is located nearly 200m away from the mall entrance, requiring an engrossing walk through a parking lot/asphalt desert to get to this "trip generator," the fact is that it is not anything special. Those coming from the central corridor are more likely to go to Hillcrest or Upper Canada, and those in Markham are likely to go to Markville or Fairview. The Promenade is hardly the regional destination that YRT would like you to believe. The ONLY exception will be once the subway is built, transit riders in Woodbridge will likely find it worthwhile. Even then, they currently have the 77 bus which goes directly there. AND EVEN THEN with the frequency planned along Jane, Vaughan Mills becomes far more attractive for transit riders.
Those which "must" go to the Promenade could always transfer from the Viva rapid transit to the 77 or 81 local buses.
3. The buses which meet at the terminal hardly operate major lines. Most could be accessed by transferring at Bathurst or Centre anyways. The only one which you couldn't transfer on to from the Viva is the Thornhill-YorkU bus, which one could meet at York University or get to from the 77 - and considering the super-local bendy nature of the line, having to transfer on to it from a local grid route rather than a rapid transit one is hardly the end of the world.
TL;DR: While on paper the Promenade routing may make sense, in reality it is forced and inconvenient.