Enormous quantities of European artillery production are finally coming online while Russia's stockpiles of artillery and vehicles are finally running dry. They won't "run out", but they can't continue pushing forwards with only what they are able to produce fresh themselves. Much like the missile strikes - sure they won't literally "run out" as long as they continue production but Russia can no longer throw 500 missiles a month at Ukraine anymore because the stockpiles are gone. Now the biggest problem is glide bombs, but that is being mitigated also.
The situation with the power grid is quite serious, though. It's going to be a very difficult winter for civilians. It's a damn shame that western nations couldn't pony up sufficient air defense before it was needed rather than waiting until after it was too late.