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?
You have the wrong concept about the 51% attack, if a person or a company have 51% of the network they could make millions of dollars, let's say they send 10,000 BTC to an exchange, and sell them, once they get the money then use the 51% attack to double spend the 10k btc transaction for the exchange, that way they recover the bitcoins that they already sold. That's the real danger of this attack.
If a state wants to force changes in the bitcoin protocol, then the community will not allow it, the community can always fork the code and fix any forced change made in the past.