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.
Would those 1.8 Million S19s be acting as one "node"? (I'm using quotes because I don't even know if that's the right term). In other words, would the other miners be able to see some giant stepping into the network trying to pull something squirrely and be able to defend it? In reality $15 Bn is chump change to the government. I could see them doing something like this and covering it up. I mean....lots of people say we never landed on the moon. I'm just a little nervous because 1% of my net worth (in 2013) has now turned into 99% of it.hahaha. I see the value, and don't trust the grimy politicians.