It is because people can launder money using bitcoin mixers. It is not rocket science. If you somehow got coins that have bad history, you can use a mixer and get different coins. The problem here is, you don’t know whose coins you are getting so the coins you’ll get might have a worse history then what you had in the first place. I wouldn’t use any crypto mixers personally for that reason alone.
There are other privacy focussed options on the blockchain which won't result in you getting tainted coins back...
You are right, a while ago I was reading an article about the same issue you referred here, it will take me some time to find the link but they were suggesting ZK-proof mechanism instead of KYC which will hide the identity of the user but help them to trace back if needed and I think only the owner could do that but after tornado was made developers has made it open source for everyone. (
source)
It's already been done, indeed it's been live for a couple of years. Basically a webapp wallet which connects to smart contracts on the blockchain which keep individual user's funds separate via zero knowledge proofs.
There's plenty of completely legitimate uses for a privacy system, rather than a mere mixer. For instance paying a lawyer; paying salaries to your workforce anonymously such that individual amounts are not disclosed; changing accounts after your original was doxed; preventing abuse, begging or harassment from people or governments due to your wealth or politics; if you've inadvertently received tainted coins; protecting your funds from the ex-wife after divorce; making charitable or political donations; trading mined crytocurrencies OTC such that the price paid is private and the settlement is entirely onchain; because you hold the perfectly reasonable opinion that KYC is invasive and encourages abuses of power etc. It's also a basic human right.
For instance railgun on Polygon allows you to create a view key to particular transactions you've made in order to prove to the authorities, if you should need to, that the usage of such was entirely legitimate. Revealing any transactions however is entirely a choice for the user, there is no admin with visibility or the ability to coerce this. You can do any of the above use cases with it, and more, or use it as just a mixer. It also seems to be much cheaper to use than a bitcoin mixer and can use any quantity of wrapped tokens.
Tornado Cash lacked the facility for users to prove their use of such was legitimate, which might go some ways to explaining the massive over the top reaction to it.
Bitcoin is not truly anonymous, it's pseudo-anonymous, that is why people use mixers to preserve their privacy. You can not stop criminals from using whatever tools are available to them. Even KYC exchanges can be used to launder money. Anti-money laundering laws aren't actually effective at stopping criminal activity. It costs more to comply with AML laws than the amount of funds confiscated from illicit activity. These costs means billions of dollars in profit for the AML compliance industry so there is an enormous financial motivation to keep these ineffective policies in place.
True, though if you know a thing or two about privacy sets and timings you can make the chances of tracing blockchain transactions rather tiny. Simplest way is to use the entirely of the DEX liquidity on the blockchain ( which is billions even for smaller ones) to mask your own transactions. For instance if I deposited my protocols own token, let's call it MyWonderfulProtocol Coin, to pay my coder's salaries with. Every month I can privately swap MWPC for Dai, USDC or USDT and transfer that to the devs. The total amount swapped would be visible, but the internal transfers to each dev would be entirely anonymous, and hidden amongst all the other stablecoin transactions. The external transaction would be mainly lost in all of the other volume surrounding MWPC, as it's wonderful and therefore has lots of volume. The devs can then withdraw and sell on a CEX without having disclosed their salaries.
If plod or the IRS demands to know why I'm putting tens of thousands of dollars a month through railgun then I can merely generate a view key for them to show that the use and payments made were entirely legitimate. Or tell them to go forth and multiply as I know that if they do take me to court I can prove compliance.
Hence it isn't that mixers are inherently a bad thing, though the fees charged seem outrageous to me, just dangerous to know or use due to their extremely limited functionality.