There are cryptocurrencies that are private-by-default, eg. Monero and ZCash. It'll be interesting to see how that evolves and whether governments go after them as well, particularly since (I suspect) Monero is used for a lot more illegal activity than Tornado.cash.

The thing about Ethereum's public ledger approach is that it enables a lot of features - broadly advertised smart contracts, for example. Tornado.cash is literally an attempt to replicate the privacy benefits of ZCash inside of Ethereum's generalized blockchain. From a computing perspective that's quite interesting - you start with a public system and find a way to emulate a privacy within it.