Gold works without the internet but it is very slow and cumbersome to ship gold across the ocean to pay for goods.
Building out 51% of the world's hashrate is probably possible, especially for a country like the USA who already has something like 40%. But it is still much harder than to just press a button and deny other countries to use the Dollar.
> What do you mean by "takeover"?
I meant a 51% attack by a state actor.