Parity in this case would likely mean nearly everyone just buys directly from steam. No doubt Sony would infect the gaming process by injecting their launcher no matter what you do, but steam will get their 30%.

I don't know what Sony could offer on their own storefront that plays well with parity rules to make people choose it over steam.

But the thought of them operating their own store front and offering steam keys with every sale won't happen. Valve have stopped that in the past.