it costs just 6 billion to destryo bitcoin with a 51% attack
Having 51% hashrate doesn't mean someone have full control over Bitcoin network. While they could reverse the blocks or attempt double-spend, the community/developers could do few things such as :
1. Change the PoW algorithm
2. Change consensus method
3. Coordinate to reject the block that comes from attacker
All of them are ugly solution, but it works and government only can hurt Bitcoin.
this is something a commited developed country easily could affort
If their goal is to prevent cryptocurrencies adoption, making laws which forbid Cryptocurrency possession/usage with severe punishment is easier, less costly and more effective.
Forcing ISP to block cryptocurrency website and control media are also more effective.
however the developed countries promote and support cryptocurrency as cryptocurrencies are benefiting them more than the developing countries, so there is a division in the united nation consulting, which prevents state sponsored 51% attacking as being legal.
They could do it secretly
however if there is an intelligent businesman who lends a ton of money to create this haspower as some form of cyberweapon of a state, this could truly exist.
No sane business man would lend their power to attack his profit source, especially since SHA-256 usually only used to mine Bitcoin.
If bitcoin were destroyed or can't be mined with SHA-256 ASIC, the ASIC won't be profitable anymore since altcoin which have biggest SHA-256 hashrate only have about 10% hashrate of Bitcoin.