If you haven't noticed, Russia has been making moves to undermine that world reserve currency, even announcing such an intention. And what backs that currency? Oil. to which Russia is the #2 exporter in the world. and they just announced a new huge oil discovery today. they are also the #1 natural gas exporter in the world.
recently they just announced a new direct pipeline from Russia to china and will do the transactions in non-USD.
You are correct that we have the worlds reserve currency, but you are wrong about "endlessly". It will end, and I don't doubt that Russia will be in the mix working to speed that process up.
I am not sure what point you are getting at but Russia still doesn't have the spending power that the United States and NATO does to fight 2 wars on 2 different fronts. Maybe in a few years they will be able to pull this off but for now, they can't afford to fight NATO in Syria AND in Ukraine at the same time.