Important Question about Privacy Token

God I love privacy token such as monero or oxen. They are a leading effort in making us use cash again. Where your government doesn't need to know that you're buying something embarrassing off the internet. But it did got me thinking.

Surely, government and tracking isn't pure evil. They must've used paper trails of transactions around the world to track criminals. But how would they, if monero became mainstream, track these bad actors? What kind of "paper" can they really "trail"?