Besides, I don't understand why everyone is focused just on ETH, for example, another player, CZ could do this with just a fraction of his wealth, and more importantly, he already has a pool set up and can grab a lot of rented hash while playing innocent.
On second thought, maybe there is a possibility that he could stay clear of the law, if only he can successfully mask all his mining activities, both before and after the attack. (I don't know exactly how easy/hard this would be.)
The point is that attackers can also rewrite the transactions of innocent traders, thus including them (involuntarily) in the steal. (This is something that we haven't yet touched upon in this thread.)
So attackers could in principle target e.g. CZ to make him look guilty as well. And on the other hand, this possibility also gives deniability to any actual accomplices. So CZ might also be able to deny having been an accomplice.
Just a thought. However, the more attackers take part in the attack, the easier it will be for them to deny their part in it afterwards, in any case.