For a given country's population, if the share of bitcoiners was the same in every country, Americans would only make 5% of all transactions. On the other hand, if using bitcoin is relative to the available wealth, the U.S. will keep on dominating bitcoin for many more years.
Except eventually they will bleed dry because America doesn't export anything of value. Countries that rely heavily on import with almost no export will be hurt in a gold or commodity based currency like Bitcoin.
This is because for every product you import, your country will lose Bitcoin.
Right now America can sustain their way of life by just printing more dollars, and the inflation will not only hit America, but the whole world. So America is basically stealing wealth from the whole world by importing goods with infinite dollars. Once Bitcoin becomes mainstream and dollars become relics of the past, America will need to find export products or else their economy will collapse, they can't print their way out of this. Even if they'd own every single Bitcoin, they minute they import something the world will demand something of equal value back, so eventually they either need to export or hand over some bitcoins.
This means wealth will theoretically become more evenly distributed. You can get a huge head starts by investing early, but once Bitcoin is the 'reserve currency' of the world you will eventually have to do something productive or accept the fact that you will eventually bleed dry.
