> If hash capacity were traded on a perfectly competitive market, then it would always make sense to rent 51% of the capacity at market rates, earn the transaction fees, and also perform a double-spending attack. There is no equilibrium point for transaction fees where this attack becomes uneconomical. The only defense is that the market for hash capacity is imperfect.
At this point in time the current hashrate of the bitcoin network is 32.500 PH/s, up from 5.000 PH/s a year ago and 1.400 PH/s two years ago. If you rent 51% of the network it's going to be rather obvious that something is happening, that will however not prevent an attack. Let us assume that you can rent capacity because the miners are greedy, what price would you have to pay? Let's assume that you can buy from miners that want to exit the mining business, so they do not care about deprecating the value of their hardware nor the bitcoin value itself.
So the assumptions are that 51% of the available capacity don't care if bitcoin tank and burn as long as they profit enough, and you're able to buy that. A 0.43% difficulty increase daily (average over last 2 years), bitcoin price of 7.600$, a 4MW powerdraw, and electricity prices of $0.08/KWh
Miners controlling 51% would profit north of $1.000.000.000 yearly, and if they just want to be compensated for that one year, you have to pay $1.000.000.000 to rent 51%. That is a lot of money, and at $20.000 high it would be tripple that value.
However, why would 51% of the capacity suddenly exit? Rather they want to be compensated for multiple years of profit, lets say 5 years and it's not unreasonable to expect bitcoin to reach $70.000 in that time. So we're looking at a $50.000.000.000 cost to coordinate the attack. That's expensive, and with that kind of money there are other ways to make them multiply. Who would pay that to ensure destruction of the thing we know as Bitcoin? After all, the success means it's likely that another *coin takes over, where you cannot 51% as easily.