If you have 51% of the network hashing power, there are two things you can do with it:
These options are mutually exclusive. You can either get rich or attack the network, but not both. There is a very strong economic incentive to not perform a 51% attack. A built-in bribe for potential attackers, if you will. Of course, a politically-motivated attacker might care more about disrupting the network than their own wealth, but really, what are the chances of that ever happening?
- Secretly (but entirely legitimately) mine more bitcoins than the rest of the network combined, making you insanely wealthy; OR
- Do a 51% attack, which cannot be done without the rest of the network knowing about, causing a mass panic and a loss of faith in the system, greatly reducing the value of any coins you mine.
These options are mutually exclusive. You can either get rich or attack the network, but not both. There is a very strong economic incentive to not perform a 51% attack. A built-in bribe for potential attackers, if you will. Of course, a politically-motivated attacker might care more about disrupting the network than their own wealth, but really, what are the chances of that ever happening?