It will never happen and unrealistic thought by the way.
There are trillions of USD in circulation, even US govt has no idea how many USD are there in circulation as they keep on printing notes every now and then (since US dollar is backed only by the US government). Today USA is super power so we see USD as global currency, may be tomorrow China takes over as super power we will see Chinese Yuan in circulation. I do agree that the centralized governments will never accept the decentralized crypto.