Who is seriously going to try and execute a 51% attack? It would be so incredibly expensive, you would have to keep spending more as people added hash power to

Who is seriously going to try and execute a 51% attack? It would be so incredibly expensive, you would have to keep spending more as people added hash power to shut you out, and most importantly if your attack worked you would instantly undermine the security of the blockchain making all the currency you've stolen worthless!