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.
Indeed. The lack of supply with high demand will always drive prices up. Happened to GTX 1070's and most of high end RX 400 and 500 too. AMD and Nvidia didn't plan those price hikes. Shortages for one reason or another did it. To get the prices back down, they need to churn out more cards like no tomorrow.