> You'd think the "decentralized finance" people would have figured out a way to separate those functions by now. But no.

But yes. I'm not sure how you've heard the term "decentralised finance" but haven't heard of decentralised exchanges, both traditional Ethereum DEXs and more novel cross chain ones. They successfully separate out:

- User funds by self custody

- protocol rules that are publicly verifiable.

- build/bring your own front end

- market making- AMM, order relayers + others

- lending and borrowing including the above stack in completely separate but composable protocols