Hi David, thanks for your reply
I understand your argument and I agree that it is theoretically possible, however, there are a few caveats.
You seem to assume that all the big miners are in the US and will comply immediately with some form of coercion from the government, but they are businesses and will not go down without a fight. Any stringent regulation will provoke an exodus from the US to more friendly jurisdictions where they could mine freely.
Something like 60% of the mining operation is done outside the US already so it wouldn´t be a big deal to start moving the other 40%.
If the hash rate drops by 50% temporarily this is not a major problem, it has happened already with the China exodus, and soon after it was business as usual.
As we are seeing now, the world is more divided than ever so a 51% seems unlikely since it would require the coordination of Russia, China, Kazakhstan, and other minor nations. That's never going to happen.
Even if somehow they managed to launch such an attack, they would have to be extremely stealthy or the network would have time to react, reorganize itself, and protect the blockchain.
Also, they would need to gather not only hash power but also thousands of nodes and these are much more distributed and loyal to the Bitcoin cause.
For the economic incentives, it doesn´t make sense and even if it was just for the sake of destroying Bitcoin, it would be extremely hard to pull off.
Worry not, we are safe.