If the United States stops providing military aid to Ukraine.... Let's be clear: The Biden administration is going to spend down all of its military aid budget by the end of its term. That means that in order for there to be additional aid, President Trump will have to request that military aid. That may be unlikely. My sense is that Russia will then move to try to win the war and will not be very interested in negotiations.
I don't really see China stepping up to increase its support for Russia. What it will do is cement the perception within Beijing that the west is not in anything for the long haul. Ergo, even if it were to get into trouble by trying to invade and occupy Taiwan and if that were to be a slog, like Russia's experience in Ukraine, after many years China would eventually prevail. The west would lose stomach for this fight, which sort of confirms the old Communist sentiment about the weak, capitalist and materialistic west. It would affirm that perception—