You can "buy cash" right now in amount from $100 to $1M on most d-web exchanges. Decentralized versions like Open Bazaar have worked for years just no one uses them so its more expensive and less choice.
Decentralized on/off ramps have existed for a LONG time and the fist was a key reason Bitcoin Core (BTC) broke away from the Bitcoin roadmap and became more of an "alt-coin" than the Bitcon we started with. Many think "SegWit" was a hack desigend to inject malicious code into Bitcoin Core to make these types of decentralized exchanges more difficult but then Vitalik just said fuck the US Government and came out with Ethereum.