You're correct that cooperation is key and that if you get "enough" people (50% of hashing power in most cases) behind a plan, they can change the rules of the game. This could be used to steal coins.

This has actually happened in the past. When massive amounts of Ethereum were stolen from the DAO, the community got together and decided that those coins did not belong to the hacker. With >50% of the network, they forked the coin and created a refund contract where people could retrieve their stolen coins.[1] The dissenters remained to form what is now Ethereum Classic.

So, what prevents this from happening in a malicious way? The first hurdle is building that consensus. Many people involved in cryptocurrencies today believe that the future value is much higher than today's value. Paying them off would not be easy, especially considering that cashing out a large sum would crash the price of the coins, and thus the profits from your maneuver.

That being said, a core assumption of cryptocurrencies is that 50%+ of the network is not malicious. Another way of looking at this is that whoever controls greater than 50% of the network cannot be considered malicious from the network's perspective. They are they consensus.

The last point I'll make is speculative. If you created (or took over) a cryptocurrency and built consensus out of malicious actors, what value do you think the outside world would place on that coin? You would win lots of coins, but would anybody pay you dollars for them?

[1] https://www.cryptocompare.com/coins/guides/the-dao-the-hack-...