Might sound dumb, but there are still places in the world where having $400k worth of BTC != $400k in the bank.
Plenty of banks will freeze your bank account instantly.
And good luck convincing them that you stumbled upon $400k by solving a puzzle - only takes one suspicious fraud/risk manager to conclude that there’s a higher chance of fraud than legitimacy.
(But you are right, no problem to find someone to pay you the market price. That’s a done deal in seconds)
Examples of such places? I did KYC/AML with multiple European banks when withdrawing crypto, and while they checked thoroughly, there were never any issues.
Plenty of banks will freeze your bank account instantly.
And good luck convincing them that you stumbled upon $400k by solving a puzzle - only takes one suspicious fraud/risk manager to conclude that there’s a higher chance of fraud than legitimacy.
(But you are right, no problem to find someone to pay you the market price. That’s a done deal in seconds)