There's a lot of hypotheticals. You can email GO to find out. But I would bet money that GO doesn't consult VIA at all on parking development. Every single one of their massive parking garages has been built because of increased GO traffic. Nothing at all to do with VIA. Heck, GO specifically prohibits parking beyond 2 days at their stations. And that includes the ones with VIA service. So no, they don't really care about what VIA thinks about parking.
How much do you think it costs to park near Union or Gare Centrale for a week?
And it does have to do with travel. It's easier to get to most VIA stations with transit, cabs, etc. Remember, city centre convenience is the selling point.