A modern reserve currency is of necessity mostly backed not by gold but by the legitimacy and reliability of a large, developed economy with an immense chain of proper accounting practices and ledgers. These assure that no one can create more currency and manipulate currency values on the sly.
Yet, even if a prospective global reserve currency is issued by a sufficiently large economy and has books that are honest and transparent and skilled manager in its central bank and treasury, much more is required of a prospective alternative to the dollar.
An alternative reserve currency must, like the dollar, be backed by an issuer that is politically stable, reliable in its agreements and undertakings, and culturally, politically, militarily, and economically dominant. The alternative reserve currency issuer also needs a combination of allies and international institutions that come to the aid and rescue of nations that intend to rely on the dollar alternative. This is where the EU and the euro fall short.
In practice, a dollar reserve reliant country that is friendly to the US benefits from a wide range of assurances and benefits from the US and its allies. They get access to aid and loans. They get access to the US and world economy on fair, even beneficial terms. Their people can travel to the US and enjoy its domestic amenities.
When troubles arise, the US and its allies will help. Not just disaster and emergency assistance, but a range of loans and grants on good terms. They also get the political, diplomatic, and military might of the US against the depredations of aggressive neighbors or internal insurgencies.
There is no 911 for countries to call, but the next best thing is that the leaders of a dollar reserve country friendly to the US can call for help and it will arrive. That is why we have a large, capable blue water navy with a massive fleet of aircraft carriers and an Army and Air force that can project power around the globe. Among other reasons, our armed forces help protect the value of the dollar by protecting the dollar's friends when they are imperiled.
What does the US get in return? The dollar's users around the world invest in Treasury securities -- which means that they finance our massive public debt. Above all, they finance Medicare, the largest single federal expenditure. Indirectly, the young men and women of our armed forces are helping to keep their grandparents healthy and housed against the burdens of age.
For all that Russia and China and the EU might envy the dollar, none of them are capable of replacing it. Above all, no one with a lick of sense trusts Russia and China not to engage in exploitative and predatory conduct at every turn if they had the world's primary currency. So the dollar endures.
*APPLAUSE* :)