I don't understand you. We are talking about a hypothetical scenario in the future where all fiat transactions are absolutely controlled by governments, no? That won't happen from today to tomorrow. As of today there are plenty of people holding Bitcoin or non-kyc Monero. In the transition, alternative forms of payment that escape the government will take hold, and those who are interested in escaping its clutches will use these means of payment.
It doesn't matter if you already have them.
If you will use it you will have to come clean and pay taxes, otherwise, you will be tracked. Of course, if you're buying drugs and spending money on escorts that's a different question but poeple were not going to talk about that even if they would want to pay taxes.
The moment you buy anything you will be tracked, car, house, rent, it doesn't matter if you can hide the transaction you can't hide the assets you've purchased, you can hide your identity when you order pizza online but when the IRS finds PizzaHut has delivered 1000 times to your address and you have zero income some might wonder where you got those $30k for pizzas. If you want to live in a tent and buy only food and soda cans from a machine yeah, it might work, but a life like that to avoid taxes? Does it make sense to you?
Also the same for your drug/arms dealer, right now they get away since the so-called control is nowhere near your absolute scenario but if it's going to be like that, what is he going to buy with the monero he received from you? There are going to be food dealers on the dark web?
At one point he will have to enter the circle of total control if that exists, and good luck from that point on.
Speaking from memory, I remember reading that in countries where cash has been practically eliminated, transactions for drugs, prostitution, etc., are made in foreign currency, dollars or euros, and cryptocurrencies.
There are three countries that are almost cashless in Europe, Sweden which is in a league on its own, Finland, and Denmark. Prostitution is legal in all of them. On the other side of the globe, Singapore also has no clear laws against it, so ....I wonder who you've read about.
And besides that if you constantly exchange your own currency to euros or USD at one point it's going to trigger a call from the IRS equivalent, you constantly buy euros, you don't travel outside and you have no euros in your bank account, it takes three clicks to track such a profile.