What will the SEC do when 0x decentralized exchanges pop up? There are already a dozen in development... pure p2p trading from a hardware wallet... https://0xproject.com/
You have to convert USD to some cryptocurrency at some point, and vice-versa, so that's what they'll regulate.
Money changed hands for me to get that currency -- money paid to the power company and to a few eBay sellers for video cards -- but nothing requiring an exchange fit-for-purpose.
If purchasing with cryptocurrency ever truly becomes a thing (yeah, I know it exists, but it's rare and becoming more rare these days), that will affect the other side, as well.