Even if they got 51% of the hashrate they would not necessarily carry out a 51% attack, as doing so would essentially prevent them from every selling their equipment (GH/s again).
That is not the problem. The problem is that for the bitcoin to get to $100,000 per btc, you need to have blind trust in the network. And that includes trusting that there isn't a shred of possibility that there is someone crazy enough to do something stupid. And we know there are a lot of crazy people.
Scenario for you: China pays GHASH $1Billion to purchase the pool for the purpose of destroying bitcoin. $1B is nothing to China -- far less that the threat bitcoin poses.
You are correct over the long term, but over the short term it is not as much of an issue.
If ghash were to receive such a payment they could simply take the payment, announce it and the miners would simply leave. They would still have the payment and the network would be secure