You're kidding me right about Afghanistan? The Sovjets were winning in Afghanistan handsomely until the US entered, created Al-Qaeda and provided them with top of the line weapons.
I wouldn't be too certain in the US economy. No one thought the USSR could fall either, but didn't the US congress not long ago actually stop paying their government employees because they couldn't agree on the debt ceiling?
The US is the one with the overstretched military now, not the Russians, which largely have a military aimed at defending its own perimeter, not project force everywhere.
So one minute you are claiming that the Russians could easily defeat the Americans and the next you are saying that it was American financing of the Afghans that caused the Russian defeat?
Spin it anyway you like. The fact remains that the Russians were thrashed and humiliated in Afghanistan.
And the American economy is much stronger than the Russian one, by a factor of at least a 1000.
Russia's whole economy depends on selling oil and gas to the EU - and they've annoyed the EU so that the EU is determinedly switching away from oil and gas. What happens then? Russia doesn't have any other industry. No IT and pharma industry like India, no manufacturing like China, nothing but vodka which no-one wants to buy.