Nothing is perfect, but once a blockchain network is big enough it becomes near impossible to overtake.

Maybe currently, if the US government really wants to, they could dedicate a few trillion dollars to take over the bitcoin network. But it would make no sense to put that much effort into what would be mostly pointless. And if bitcoin ever reaches a point where it is a full on threat to the dollar then it’s network would probably be too big for any nation to overtake alone.

So you are not wrong in theory, but in practice it is near impossible.