You seem to see in extremes. The move away from US will be gradual. So maybe some of oil is conducted in Euro and some in RMB.
The Arabs will not move 1 Trillion from US to Euro just like the Chinese will not dump their Treasuries. Arabs will probably start by accepting Euros and meanwhile attempt to reduce their Treasury holdings into gold, US assets, and other assetsetc. They shifting out of US Treasuries to non US$ assets will further weaken US$ and the chain reaction continues. Along the way people like Soros will place a big bet on the US$ weakening even more and reap billions (he knows that the big holders cannot move fast and will lose too much if things move too fast - Chinese might even come in to defend the US$!).
Everything will be gradual.
After all the Arabs have lots of things to buy from China in terms of -the usual items that the rest of the world buys - clothing, toys, electronics, furniture etc etc etc. So they would not mind holding RMB. They might hold onto some Euro because they buy lots of aircraft, weapons, industrial tooling, travel (since US is not that welcoming of Arabs) and they will probably hold some US$ (US weapons, aircraft, software) too.
So the move will be gradual. As the US economy gets itself into more debt, its finances become more shakey. Who would accept an IOU from a person on the verge of bankruptcy?
Do you mean that you cannot see a situation where people stop buying US debt or demand higher returns for US debt should the US fiscal situation contine to weaken? When the Chinese start asking for more returns, that high level of interest will impact US economy. And with the high levels of US debt, the politicians would like to devalue the US$ to burn off that debt since high inflation is good for debt (bad for the debt holders too).
And the situation is gradual on the debtholder's side too. US$ is not going to go to 0. It will weaken, for example 10 to 15% per year. And as it continues to weaken, the Chinese will ask for more returns and or start to diversify out of Treasuries into maybe US real estate, US farm land or US ag companies.
Throw in 10 years and I can see the US slowling losing its reserve currency role and RMB with another basket of currency forming a new reserve currency. But it will be gradual.
And guess what, if the US puts its financial house in order the US$ may still be a dominant currency. However that means painful cuts on expenditure. It means leaving Japan, South Korea, reducing it military and as such have a smaller global footprint, recognizing that it is not longer a military superpower/global police. It will have to reduce services to its citizens.
I remembered when Ringgit and S$ was at parity. But slowly as Ringgit weakened, people slowly stop accepting Ringgit. At 10% differential, many still accept the coins but when it got larger, even Malaysian coins were not wanted.