It'd only take a few of the oil producing nations to start trading in a different currency. Back in the year 2000 one of those countries started trading oil in Euros. That country was Iraq. What was one of the first actions taken by the US after the US asserted control over Iraq? Replacing the Euro with US Dollars for oil sales.

If one country doing this is enough to spook the US, what does that tell you? Furthermore, this has not been an isolated incident, other oil-rich countries have threatened to leave the US Dollar behind and have faced instability shortly after (Libya, Syria, etc...).