He'd have to sell all his BTC first, subsequently crashing the BTC market value because of too much supply. He'd be lucky if he even got half of the total market value for it, because of massive price drop.

I am not sure why he would do that. A major whale knows that if he wants to sell, he should do so in small batches to keep the price from crashing, or else he won't be able to sell for the price that he likes.