Highway expansion doesn't rule out arterial road expansion. If anything highways encourage bigger arterial roads because of how they concentrate traffic. The theory that added road capacity encourage more driving does apply to areas where everyone already drives. Not because people are switching to cars from other modes, but because more road space encourages more trips and longer trips.
In this case we must be specific...theory is utterly useless. A highway would simply not encourage more driving than widened roads would. It makes no sense in a place where everyone already drives, where no travel is prevented by congestion (should either a highway extension or widened roads be built), etc. Are people going to drive to work twice a day? No. Will more tourists visit the area? Possibly, but then there'd be road capacity freed up somewhere else outside the city where no transit/congestion affects auto trips, either. Even if we dream up theoretical families that walk to the grocery store in Keswick but would drive to Newmarket instead or highway-dependent industries that move to Keswick, we're talking about a literally insignificant and unmeasurable quantity of "new" trips and not a reason to oppose a highway in lieu of road widenings. Widened roads can add more road space than a highway, anyway.
Also, you were talking about highways accelerating growth, not road space in general, so don't mix the two up. I would clearly not dispute that added road space can generate trips because roads in northern York Region are becoming increasingly congested and will eventually this will impact leisure/shopping trips, location of businesses, etc.
I was originally responding to urbanfan89's erroneous belief that all (or practically all) of this growth is dependent on highway construction. It's not, but even if it was, Georgina would continue to grow. The only thing that mildly supports him is that the Keswick business park may not be permitted without the highway extension (the rest of the planned growth, including all residential targets, is not 404-dependent)...but if it was permitted and served by widened roads, it's also purely theoretical that a highway instead would result in more trips than just, say, a 6-lane Woodbine.
Even if both a highway extension and widened roads were built, the greenbelt and official plans are in place to dictate how much growth occurs. Building both would mean more pressure but hopefully that pressure can be contained as legislated. East Gwillimbury plans to widen Woodbine to 4 lanes and keep Leslie at 2, but this plan assumes the highway(s) is(are) built. If they wish to grow in the event the highway(s) isn't(aren't) built, they'll have to widen the roads even further - no highway probably means more limited employment sites (the Keswick business park) which means more commuters barrelling down streets like Leslie or Warden. You claim that it'd trigger longer trips, but since Georgina's residential targets are not 404-dependent, building the extension would actually result in *shorter* trips, especially if Queensville is built with enough retail/employment to preempt people from going farther south.
It's kinda ironic that the 404 extension would help an exurban place like northern York Region shed it's bedroom community status. The Bradford Bypass is a whole other can of worms, though.