This is essentially the purpose of localmonero and similar offerings. Trading cash for Monero in a p2p manner is going to be extraordinarily difficult to halt.
First, the chain only sees the monero side of the transaction, not the fiat side, of which it's likely that no records exist at all after a short while. It looks identical to a payment for a good or service and it also looks identical to a transfer between two of the same person's wallets.
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.
This is essentially the purpose of localmonero and similar offerings. Trading cash for Monero in a p2p manner is going to be extraordinarily difficult to halt.