What happens when you stop your 51% attack? The participants whose party you ruined can simply roll back the chain to before your attack and continue business as usual. 51% attacks are expensive because they must be sustained. Sure, you could try to take the coins and exchange them quickly, but if you attacked a coin with a big enough community (Bitcoin) the message would spread so fast that there would be a coordinated effort from stopping you cashing out. If someone just wanted to destroy a coin without caring to exchange for their currency of choice (like a government), well, even they would have to sustain a perpetual goose chase since people can just abandon the attacked chain and take up another and only provide support to the chain(s) that are not being attacked. It would take word of mouth, but we are using Internet where messages spread to millions with seconds.