I agree with what you're saying in how the US government can and will use intimidation and will act on, or coerce other governments to act on foreign companies and individuals. However, if even one jurisdiction refuses SEC demands, which is increasingly likely as BRICS nations pivot from SWIFT, and allows an unregulated exchange, what now? WikiLeaks is still kicking and Julian Assange isn't in jail (supposedly), and we all know to what ends the US has gone to plug those leaks.

My other point also remains, how can a government, or coalition of governments take on a decentralized system without it being an exercise in futility?

The proverbial monkeys are still torrenting en masse, despite the killing of and threats to kill more proverbial chickens. Given .gov success at taking down torrent sites, ahem ThePirateBay, and prevention of peer to peer file sharing, what success will they have at taking down decentralized cryptocurrency exchanges?