I hear a lot about how safe bitcoin is. And 51% attacking it for a private person/ company does not make any sense and is unlikely.
But what would happen if a state wants to attack Bitcoin and uses a vast amount of compute budget (in the billions). What would happen?
Can you understand that in order to do 51% attack they would have to either buy some of the pools to redirect their power, either buy as many mining devices as needed to match the current power (let's approximate to buying as many mining devices as they are in use now)? Or, well, a mix of the two. I myself I'm unsure of the real magnitude of this number (of devices).
The thing is that even big mining companies may have to stay in the waiting list to buy the gear they need. So it would be a very slow and expensive task, not mentioning that since those billions would be taxpayers' money, if the government is a democracy, it will be the last thing (use of money) they'll approve before getting changed and the preorders canceled.
Plus even if they were successful in buying all the mining hardware required, and actually did a 51% attack, the community, the economic majority, and its army of full nodes would make an emergency hard fork and kick the attacker out of the network. The state-level actor spend billions for a mere double-spend.
But we could be very VERY confident that during the time the state-level actor was accumulating the hardware for its attack, it will realize that what's keeping the network together is its incentive structure, and that it's actually better to be an honest actor in the network and be incentivized in Bitcoin.