Following scenario: A government would like to destroy Bitcoin. It buys a significant number of ASICs to build up a huge mining farm and carries out a 51% attack.
First thing, why would a government do that?
And if they are afraid of bitcoin, it means they should be afraid of ethereum once bitcoin is gone, and next about litecoin and them some PoS coins and so on and on. From a government point totally wanting to get rid of cryptos, it makes zero sense to start spending billions in order to pluck only one at a time.
And why kill bitcoin entirely, they could deal with the problem in their country only.
If they are that stupid to plan such an attack first they would put lifetime sentences as punishment people using bitcoin, how many do you think would risk it?
From my point of view it will not be so easy to buy so many ASICs as once - what do you think?
In order to get 51%, you would need roughly 2 million s17, taking into account that while you set them up some miners will drop because of the reward getting smaller, maybe you can do it with 1,2 - 1,5 million, but I doubt Bitmain or Cannan or all the manufacturer together would be able to produce that in a short period of time.
Another question: Will it be noticed (how?) that somebody buys a big stack of ASICs and/or are we able to identify such a behavior monitoring the network's hashrate?
If you don't have an insider at the company selling, you won't know it till the hash rate grows and since the hash rate is normally on an upward trend right now because of better gear and the price increase you will need really good clairvoyance skills to realize it's an evil player adding hash power to the network.