[Sohcahtoa82]

I think it's a matter of how quickly the thief could make their profits before people notice.

Of course, keep in mind that for such an attack to work, you need 51% of the total ETH being staked. Again, that's billions of dollars right now. If you stole some coins via double-spend, you couldn't pull the money into usable fiat very quickly. Even dumping a bunch of coins is going to crash the price.