There are plenty of things in ubiquitous use that are illegal in some places, or even most places. Alcohol is ubiquitous, guns are everywhere, gambling, cocaine and marijuana come to mind, people modify their cars beyond legality quite often. Lots of people do illegal financial activity as well, most people transact under the table when the opportunity arises. You can have a scenario where cryptocurrencies are widely used even in places that ban them but are so ubiquitous that enforcement is untenable.