Btw, crypto (like bitcoin) is only an alternative because of convention.
The complete history of bitcoins is globally trackable, and people could all decide that they'll pay more for bitcoins that came from Satoshi's initial hoard, or that they'll refuse to accept bitcoins that were ever seized by the FBI.
(Yes, there are mixers. But you'd just refuse to accept any bitcoin that took part in the mixer transaction, if any FBI coins were in there.)
I'd like to introduce you to Monero, which isn't globally trackable and also properly fungible so you can't refuse mixed transactions (since all transactions are protected).
Apparently it powered online drug marketplaces before Bitcoin existed.
Try doing that with crypto. Who are you going to arrest?
Every on- and off-ramp provider. EU legislation has basically created a database of real person to wallet mappings (for some subset of wallets). You can't take money from a wallet if you don't know who it belongs to (if you're an exchange anyways). The checks are a bit soft (ie. self attestation and stuff), but the public ledger part of crypto makes tracking far-far easier than with traditional banks.
The end game for this is that people in the West (and whoever they can pressure) won't be able to buy crypto to buy drugs or sell it when selling drugs, making it useless on a big scale.
Second, Monero is still thought to be untraceable. In fact regulated entities are banned from exchanging it in the EU precisely because they can't trace it. (Zcash is also banned under the same law, but is considered technically inferior because not all transactions are private.)
Third, what do you even mean? Do you mean they'll go back to the last time those coins passed through a regulated on-ramp, and prosecute that person? For what? Buying cryptocurrency, then buying a legal product with cryptocurrency, is not illegal, and even if the product was illegal, the government most likely couldn't prove that. Also, the on-ramp was probably in a different jurisdiction. Perhaps for something like "acting as an unlicensed money transmitter" which is a thing they have done against users of cryptocurrencies. If they prosecute that in large quantities, will it fly?
Or do you mean they'll wait until someone takes the crypto to a regulated off-ramp, and then prosecute that person? For what - undeclared income? As far as I know, trading one cryptocurrency for another is a non-taxable currency exchange, at least in some EU countries, so they can't get you for that. And what if they declared it? Again, they might try "acting as an unlicensed money transmitter" of course. What if it never gets to a regulated off-ramp and just circulates peer-to-peer forever? It's more likely tyou think, since remember, regulated off-ramps are strictly banned.
Why do you believe it would suddenly make peer2peer cash to cryptocurrency exchanges unviable?
And if you meant tracing the Monero itself, I suggest you read up on how Monero works—and how it differs from BTC—first.