Trade deficits.
Think Europe or China will ever run deficits large enough for long enough to function as a reserve currency?
What is happening are barter deals: oil or some raw material to China who pays in Yuan and then the Yuan pay for imports from China. Or better, China makes big loans, Chinese and local officials enjoy a feast of embezzlement, and the country reneges on the deal.
A reserve currency is not based upon your ability to inflate it away. That’s exactly what got us into this mess. China and much of the world wants something more stable. They don’t wanna store their wealth in dollars and watch it’s value erode away by inflation which we used paper massive military and welfare programs.