Why do you asume that they will always revert and invest in the US??
I don't think anyone here is assuming that the investments will always revert to the dollar. In fact a partial basis of Ripley's concern appears to be that eventually they will not.

My point was that the reversion to investment in the dollar assets is what they have been doing. That is an investment choice and has nothing to do with the currency oil is priced in. Even if oil is priced and paid for in dollars, oil producing nations can convert and invest in Euro assets.