The laws will be trivial to enforce. Expect to be unable to exchange cryptocurrency (that you own in your wallet) to fiat, which will almost kill the cryptocurrency market for anything other than speculation.

I expect a parallel, dark economy will still be around, where people transact with cryptocurrency and can only exchange it for cash.

It will kill the speculative market. Their native use cases in their own systems will still be 100% viable. If someone uses for example bitcoin cash or miner as money as they're intended nobody can do a thing about it. For those people uninterested in off ramps this means little.

I use it as money as it's intended, to get paid for my work. The supermarket doesn't accept BTC, does that still make it 100% viable?

You better believe I'm going to be paid in USD from now on.

For those people who use it, yes.

I'm not so sure. Unless an EU citizen wants to risk submitting a false tax return under penalty of perjury, any DeFi money that results in a profit has to be reported elsewhere. Your bank will probably also start to ask questions if you do enough DeFi activity. You can probably get away with small transactions, but is it honestly worth the risk?

I don't see this law as impossible to enforce, but rather, difficult to enforce. The government will still try, and most likely succeed in catching several institutions that don't comply. Governments don't actually need thorough enforcement for laws to be effective.

Being easily capable of breaking a law does not mean that the law is unenforceable.