It’s very unclear to me if you actually understood the discussion about forex fees.
One does not “solve” forex by trading in a common shared currency, that is already possible without stablecoins - it’s just that vendors often want their local currency and therein lies the problem.
Even if everyone moved to stable coins or Monopoly money, once you need to cash out in IRL, you encounter forex fees.
A sort-of example is Trump’s insistence on oil being traded purely in USD for the USA’s hegemonic benefit - but other nations are waking up and opposing this status quo actively.