We do see it though. There's Uniswap and Bancor on ETH. The trading volumes are in the billions of United States dollars per day. There is also Bisq as a decentralized fiat onramp for BTC and XMR (and soon on-chain atomic swaps). Today it is possible to completely interact with the crypto ecosystem without centralized exchanges.
If you're worried about the code changing, you can run e.g. ETH nodes yourself, download the DEX frontends, and run the DEX locally.
Or you can visit uniswap.eth if you have metamask installed since it resolves ENS domains.
Except how do you go from crypto to USD on a decentralized exchange lol... You don't