American Truck Simulator
18 days 19 hours
13 days 10 hours
Grand Theft Auto V
17 days 16 hours
Farming Simulator 15
11 days 5 hours
Separate dedicated mining cards already exists (not for Vega though) but that doesn't change a thing. And there's no way to regulate it.
A large stock of the same GPU can be needed for all kinds of things, not only for mining.
For example, a lot of GPU's could be needed for rendering. Or for an internet café, or a place for kids and young adults to spend time at after school and evenings. Or all kinds of other things.
Sure, only making it possible for a company to buy larger stock could go a bit of the way, but seeing it's probably easy to just create a company (at least in Sweden, and doesn't cost a lot more than just above a hundred pounds), that's not a proper solution either.
The only real solution is to produce enough GPU's that the market (both mining and gaming) needs.