Well even if the Richmond Hill and Stouffville GO lines are upgraded to all-day service (a long way off despite ridership, unfortunately), I'd say it's still not a great solution for a lot of the people who currently use the TTC to go from the 905 to the downtown area.
Why, you ask?
Because if you go to U of T and need to get to College St, or if you want to get to Bloor or any other part of Toronto that isn't between the Lake and King Street, you'd have to take the TTC northbound to your destination anyway. This eliminates any cost benefits of avoiding a double-fare by taking the GO, and it makes people just use the TTC from the beginning instead of the GO.