Russia+NKorea surely can't go longer than US and Europe, look at the numbers in economy, population etc. Even if the US pulls out, Europe on its own would have more than enough resources to outspend Putin. It's a matter of political will, that's all. If Europe would be equally commited to Ukraine then Russia wouldn't stand a chance. So whether it's a waste of money or not now is a question of whether Europe is willing to properly support Ukraine. I guess you could make the point that it's about time we finally decide this, but the means surely are there and at our disposal.

Russia actually can't go on forever. There's a reason they're having to buy bottom of the barrel artillery shells from North Korea of all places. They've had gigantic reserves from the cold war to burn through, but they have been burning through it.