Your logic would be working if Russia was actually taking land and holding it. They have failed at that. Ukraine has taken back land or Russia has seized for short time and then loss land.
Most of the major cities are still held by Ukraine: Kiev, Kharkov, Lviv, Odessa, etc. Take a list of Ukraine's major cities. Only Sevastopol, Mariupol, Luhansk, Kherson, and Simferopol are in Russia hands and most of those were either taken in the first month of the all-out Russian invasion or in 2014 in the Annexation of Crimea. No major city has been captured the last 2 years.
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EDIT: The best bet for Russia would be to hope the West grows tired and stops the flow of aid. Ukraine's greatest threat is the growing US National Debt issues. USA is eventually going to have to account for its debt.