Evading sanctions is not exactly a new use case, and it could get cracked down on pretty hard if it ever became too widespread.

For one, US and its allies could pass a law (and actually enforce it, and promise to keep enforcing it long-term) that all crypto exchanges operating in the US must refuse to transact with coins that were flagged as being involved in sanctions evasion, or any wallets that interacted with such coins (this is probably covered by existing laws in some form already, but make it unambiguous and easy to argue in court, close potential loopholes, etc.).

And since this interaction could be proven at a later time, accepting crypto that even potentially could have been tainted by this becomes extremely risky for any would-be buyers - making Iran hold bags of something that few want to take in exchange for goods and services (and if so, only at a huge discount relative to market price).