If bitcoin is popular in 20-30 years, then there will always be shadow markets in the world.
In Russia, there are no problems with exchanging cryptocurrencies for fiat and vice versa, and in other countries there are groups in telegram channels where you can exchange cryptocurrencies for fiat. There are many exchange offices in Turkey.
And how would you exchange $1M in Bitcoin for fiat? You live in France, go with your Bitcoins to Turkey and come back to France on the plane with $1M? There is a limit of €10,000 to travel in Europe. The same applies to Telegram groups, etc.
Changing or spending small undeclared amounts is not a problem, when the amount starts to be big it starts to be a problem, and the bigger the amount the bigger the problem.