i think they'd rather shutdown and sell their BTC rather than helping crashing Bitcoin price.
Shutting down would do more damage to the network security (and in turn to the price) than obeying the government rules.
This big mining companies care only about FIAT profits and they will keep operating with OFAC rules rather than shut the whole operation down.
I expect performing 51% attack would have more and quicker impact on Bitcoin price, rather than reduced hashrate though.
I would say such attack isn't profitable, rather than fairy-tale zone. And FWIW, Top 500 list exists.
Also last time I have made calculations on #1 fastest supercomputer in the world it was not enough to do 51% attack.
What
if multiple company or government who own those supercomputer wouldn't collude together.
And all they could do is double spend, censoring transactions would not work because the amounts, sender and receiver are encrypted.
They theoretically could prevent all transaction from getting confirmed, by only mining empty block.
On Bitcoin it's much more devastating and we have to believe that two or three of this companies don't collude together because that's all it takes to 51% attack in this centralized network.
I would question about the centralized part, but otherwise it's true. But don't forget they're risking their long-term reputation, long-term profit and potential law-suit.