Perhaps China could decide that Satoshi's blocks should be given away to charity.
Have you got any idea of how much energy it would take to reach satoshi's blocks?
4. What would this do the broader cryptocurrency sector?
It would probably lead to total collapse for years.
5. Have major governments e.g. the US and NATO countries ever (publicly) announced any contingency planning around this problem? Putting a $1.5T asset at risk is something that could rattle the entire world economy. This seems like something they would at least think through?
Right now? Not exactly. However, with the passage of time, we're witnessing an increasing number of powerful entities seeking a share of the pie. For instance, Blackrock recently made investments worth billions of dollars. Do you believe that Blackrock, the world's largest asset manager with over $10 trillion in assets, would tolerate another country attempting to undermine their own assets like that?