It takes 3 seconds to look up the 24h volume for BTC and it is $9.6 Billion
https://news.ycombinator.com/newsguidelines.html
"Be kind. Don't be snarky. Converse curiously; don't cross-examine. Edit out swipes."
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)
Liquidity of less than a million worth of BTC hasn't been a problem for a long time
Then again, it may be serious trouble if you have serious issues explaining the source.
2. Deposit bitcoin
3. Market order sell all
4. Withdraw USD
Step 2.6 Coinbase blocks your account citing "Suspicious activity"
Step 2.7 Sign up to three other exchanges, split the loot across them
Step 5 Argue with the bank about the source of funds
Step 6 Argue with tax agency that you'll fill out your taxes correctly
Taxing this is super simple, you just mark as "other" - akin to finding money on the ground.
Arguing with bank - which specific bank would cause problems here?