The only way they make money (cash out) is if trust remains in the bitcoin platform. By doing 51%, they tank the platform itself. No liquidity. Provable and transparent majority attack is MAD in the strictest sense.
Not if their cash isn't - somewhat ironically - tied up in BTC.
Really, I'm sure between the two of us, we could imagine a thousand and one viable ways of making a tonne of cash very, very quickly that wouldn't involve transacting in BTC at any point!