Who will they trade with? Themselves? Will Saudi Arabia join in when it means the US abandons them, stops selling weapons, and takes other punitive economic measures? Cryptocurrency is a much greater "threat" to these systems, but even then it's more of a threat to countries with poor quality or unstable currencies.
> when the Greenback is no longer the reserve currency.
It's just a fancy title. Dollar is the reserve currency, but Euro, Pound, Yen, etc. are all reserve currencies too. It's not some sort of designation bestowed upon the dollar from which the downfall of America will lead to it losing such status, it's just what central banks and governments hold in reserve. If the US dollar loses market share (if you'll allow here), it'll probably drive up the value of dollars since the US federal reserve no longer has to print dollars to help maintain global liquidity. The whole RESERVE CURRENCY thing is just FUD.