Every sane person has known for a very long time that the dollar must collapse someday;
Not anytime soon. No universally-recognized replacement on the horizon.
Why do you assume that a replacement must exist
before it collapses!?
Because its value is rooted in psychology. Right now people sell all types of assets (even gold) to buy dollars. Completely illogical, but that's a reality. Dollar will collapse when people will lose faith in it, when they'll rush to sell dollars to buy something else. That "something else" will become a new universally-recognized replacement.
Lots of worldwide value is locked in dollars right now. That value will not disappear. It will flow into "something else", into a replacement vessel.