it doesn't matter how much anyone gobbles up. When we talk about Bitcoin being decentralized we're not talking about the circulating supply. The US could buy 90% of the Bitcoin and it wouldn't matter at all.

Bitcoin rewards net producers and punishes net consumers. No one who is a net consumer can hold onto his Bitcoin.

If the US buys 99% of it, the remaining 1% can be used by everyone else. Still everyone's money. Over time the United States, the world's largest net consumer, will be forced to give away it's Bitcoin in exchange for the things it wants.