Russia had a lot of resources and a lot of territory
Russian partisans were extremely effective. Yeah, American equipment was critical. Doesn't mean they couldn't have found another way to pull it off.
And anyhow lend lease began before America's entry into the war. If you want to postulate no entry *and* no lend lease, then that's a scenario that I have not even been discussing.
As I said, I've used computer games to model exactly this situation. In them I've completely subtracted America's influence on the war. It's a near thing, and part of the issue is that the AI is not as good a general as I am. But then how good was Hitler, really?