The Dollar won't work without internet either. Unless you put dollar notes on a ship and send it across the ocean. But that is extremely slow and cumbersome. And having that ship shot down and the notes destroyed is even worse.
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"?