in is all in your head thinking such attacks are possible and that is because of your lack of understanding of bitcoin.
first of all it will take a lot more than just $6 billion to 51% attack bitcoin not to mention that you would have to add hashrate to the current existing hashrate effectively increasing it by a lot which makes the total hashrate and 51% of it a lot more than it currently is and the cost will increase by it also.
no nation in the world is going to dedicate such huge amounts of funds to attack something they can not even destroy which brings us to secondly.
secondly 51% attack is not going to destroy bitcoin! it is so easy to fight it. the most extreme solution is to simply switch algorithm. after all an extreme $10-$20 billion attack needs extreme fight backs. in short in your theory a nation wastes a HUGE amount of money to attack bitcoin for 1 day tops and cause disturbance for a short amount of time and then all their efforts will become obsolete in 24 hours.
i think it will be necessary to attack and destroy bitcoin, bitcoin will ultimately destroy itself due to its unability to fix issues. its just a propaganda coin along with many other cryptocurrencies.
why doesnt anyone use certified paper money called papercoin, or zertificated dollar?
because empty money loses its trust sooner or later.
if you truly believed that then you wouldn't have been here on a bitcoin forum, on a bitcoin board constantly talking about bitcoin.
I really believe Ethereum is a pure shitcoin, you don't see me go on an ethereum forum like forum.ethereum.org waste my time creating an account and waste my time talking about it constantly! there is nothing in it for me.
i missed a word "not" in my last post,
ohm jes i doubt on bitcoins long term ability to survive and go on.
same with many other coins,
just look at zcash or dash, they lost 85% or bicoin gold which lost 92% of its value when it hyped, the market disintegrated gets more and more diverse and confusing.
there might even be a new dynamic, of the creation of new markets and indexation of new markets, as there is no rule of law that defines a market.
this forum for now is used also as a project development plattform, this might also soon move somewhere else.
regards