Suppose you have 100 BTC and price is $100k per coin, and you want to live a $10million lifestyle. You spend $1million on a nice condo and $100k on a car, you sell enough to make a decent yearly income from dividends, and so on... do you think the government will not ask where the money came from? because they will and they ask for origin of funds. What will you say and what data you have to back it up?
Lots of peeps do it. It's not that hard to do. One could simply bounce it from one country to another, set up a business, and a bunch of other things. You don't need to be a billionaire to do it.
If you decide to put your fiat money in the system, it's already clean by then, and if you put it to work earning dividends, then you are paying taxes on that income, the government will likely not go after you. They have bigger fish to fry.