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.
If you make a longer chain than the rest of the network, then self-interested miners are going to switch to mining on top of your chain instead of risking their work on the old chain being for nothing. If the attack is short, then surely most miners won't be upgraded to be picky about chains in the time after the attack, and everyone else will know that and won't bother trying to make their miners prefer a losing chain.
If the point of the 51% attack is profit-oriented, then the attacker is going to do it long enough to get their double-spend confirmed and then stop doing the 51% attack.
