How difficult (expensive, realistic, etc) would it be for the US gubmint to buy up enough mining power to hijack the network, change the rules, and send it all to zero?
An S19 Pro costs about $8k and has a hash rate of 110 TH/s. For a 51% attack, at least 200 million TH/s is needed, so about 1.8 million S19s, with a cost of about $15 billion. There is also the cost of the supporting infrastructure, labor and electricity.
As far as your concern about changing the rules -- it is not possible. Changing the rules would simply cause a fork, like BCH, BSV, and BTG, that can be ignored.
The most likely attack would be a denial of service. The government would prevent other miners from adding blocks to the chain and publish empty blocks for as long as needed in order to kill the network. In the end it would be expensive, but it is probably doable if there is sufficient political will behind it.
Considering that $15B then i could say that it is really that doable if they are really that serious on taking Bitcoin down but it seems no one or they arent getting that support on doing so in speaking on giving out
a network attack and minding off on how the hell they would really be spending out on something which it cant even assure if it would totally kill down the network or just giving out some temporal effects
but not totally a reason on making it dead? For sure there wont even how filthy rich are those people who do tend to support such agenda but well we dont know on whats coming though.