The problem still comes down the problem that Ukraine is far larger than the Russian Army can occupy. Ok, so they might be able to advance, but with the Ukrainians attacking their supply lines, they will sooner or later end up beyond their supply heads, and subject to small counter attacks. The Ukrainians still have plenty of T64's and T72's on hand, even as their artillery supply dries up. The broken tanks they can dig in as pillboxes. The Russians ultimately will go nowhere. The best they can