The only thing that prevents the euro from being the main reserve and world currency is that there is already one, and it is extremely inconvenient, and difficult in practice, to move away from an existing standard - see how the sterling is still used in some commodity markets. But if the US keeps on trashing its currency, there will be a steady build up of the role of the euro, and at some point a crisis might tip things over brutally.

The fact is that there is an alternative to the dollar, so in the even that people decide to run away from the dollar, there is no question as to where they will go.

(And no, I don't believe the yuan/renminbi can play the role)

In the long run, we're all dead. John Maynard Keynes