This topic has been discussed so rigorously and the conclusion is always that no reasonable state or adversary would spend so much resources on trying to stop something so insignificant. Unless you're looking for any sorts of profits, 51% won't work. You are bound to spend a few billions in both the hardware, facilities, electricity and various other miscellaneous costs for such an insignificant attack. The money could be better used in other areas. You COULD diminish the confidence that people has in Bitcoin but another crypto would simply rise again and the cycle continues. Not to mention that ASICs are not that easy to obtain or operate in bulk.
What would also kick in is what I just call the "Hydra-phenomenon" for now. If you attack Bitcoin and succeed in rewriting a block or two and thereby harm the network or even destroy it, that is not going to be the end of decentralized currencies. So even a state with malicious intent and the will to lose billions of dollars is aware of the fact that firstly, there are already a ton of other networks that must be attacked as well and secondly, if the first attack on Bitcoin is enough to also tear down all other cryptos, a new protocol with security adjustments will definitely appear shortly after. I think Bitcoin is just a genius invention particularly because it is open source and you could tweak all kinds of aspects about it to get it going again even in another version.