There are projects like Miximus, which is an Eth tumbler with no operator. Participants deposit funds into a smart contract, then when they want to withdraw later, they use zero-knowledge proofs to show that they know one of the private keys without revealing which one.

There's also Zcash, which applies similar logic to all shielded transactions.

Presumably the people who use tumblers never transfer such a huge amount of BTC into any one single tumbler, at a time, that would cause the tumbler operator to seriously contemplate pulling an exit scam.

Tumble it in small amounts at a time. Maybe the tumbler runs away with a few hundred thousand but you are still left with millions.

You'll have potentially thousands of criminals trying to get their money back? I don't like those odds but if someone wants to try that go ahead.

it seems logical to me a sophisticated syndicate would run their own tumbler. I'd assume there is some kind of OSS wordpress for mixing out there. And I don't think it'd be that hard to get other people to use it to get liquidity (just offer it as a free service).

Well, of course another will come along!