And the dollar won't be the world reserve currency forever.
I think you can look at anything and say that it will not be the case forever and be guaranteed to be right. However, the USD, despite its issues, is looking strong for the next few decades because the other currencies have even bigger issues than the greenback.
The Euro's speculative value is based heavily on the strength of the union, rather than the strength of the large economies backing it. No matter how strong and stable Germany and France may be, they can never wholly support the Euro's worth. Even with a strong Italy and Spain, the Euro would still be prone to shocks and fears. The Euro is held as a reserve currency, but not with the same gusto as it was 5 years ago.
Chinese Yuan RMB, is first and foremost, a controlled currency. Currency exchanges in China are limited by law and complicated to perform, leading it to have a lower volume of trade and thus lower fluidity on the foreign market than Australian/Canadian Dollars, Swiss Franks and Mexican Pesos. Beyond that, it exists to serve Chinese monetary policy, rather than American monetary policy largely existing to serve the Dollar, leading to a lot of exposure to holders of large amounts of it.
The Japanese Yen is used as a reserve currency because of its low inflation. But because of its low inflation, Japan is not growing, despite the 0.1% interest rate they have had for almost a decade now, which makes it unlikely to eclipse anyone anytime soon.
Australian dollars, Swiss Franks and Canadian Dollars, each are underpinned by economies too small and too focused on particular sectors. The only reason AUD and CHF are traded so much is that they move rapidly in a predicable direction due to certain economic conditions. It makes them suitable for a hedge in currency baskets, but not to hold the bulk of value. Besides, its not certain that there is enough of these currencies in circulation to make holding the world's reserves in it practical. Possibly Australia and Canada are just large enough to endure foreign banks either hording their currencies or creating money on paper by lending the another currency against debts in their own, but Switzerland with its half trillion GDP would surely consider this as an act of sabotage.
So what does that leave beyond USD? Pound Stirling, which is in fact a commonly held reserve currency. But the UK does not have the natural resources or the population for rapid growth, so I doubt it will overtake the US.
So what is left? Brazil, Russia and India? Well, the Ruble is a basket case, the Real has 8.5% inflation and the Rupee is in deflation at the moment, leading to concerns about money supply.
I think the USD could get a lot worse before people start thinking about getting rid of it.