Some GDP numbers to put things in perspective (source: tradingeconomics.com).
Ukraine: $0.2T
Russia: $2T
United States: $27T
European Union: $18T
So even if you drop the US, only the EU should be sufficient. Russia
More specifically, I saw estimates that Russia spends about 20% of its GDP on the war. This is sort of in line with estimates from the second world war where numbers, from the top of my head, went up to 40% or higher. If 20% of your economy is busy throwing ordnance at someone else sounds pretty unsustainable, then you are probably right. Inflation in Russia is now at about 20% to 30% [1].
And the kicker is that the EU only has to spend about 2% to match Russia's numbers. Long-term it's just a losing proposition for Russia. Their economy is tanking while the rest of the West just gets stronger on an exponential rate. Even if the EU would spend twice as much as Russia, then that sounds perfectly sustainable for the EU.