I think there are many misunderstandings about a 51% attack. There isn't much you stand to gain by using 51% of the hashing power to double spend. Let me explain...

There can only be one entity at a time who is in a position to do a 51% attack. It's not like everyone you transact with is suddenly going to burn you with a double spend. All the honest miners will focus their efforts on identifying who this 51% attacker is and not doing trades with them. Once it's known the network is under a 51% attack, the honest nodes will quickly start blacklisting addresses that performed double spends and limit the attackers ability to spend his coins. He would essentially be destroying all his coins.