About 40% of Russia’s revenue comes from oil exports. Their currency is heavily tied to it. It will kill the strength of the Ruble. They will have to either drastically cut spending, print money, or borrow from other countries. Put increased sanctions on any country that buys Russian debt. This will skyrocket inflation even higher from the 9% it is at right now, collapse the economy, and leave Putin in horrible political standing amongst his people.
Putin is all about his legacy and long lasting imprint on Russia. I think he’d rather have the Russian propaganda history books say that he “successfully” invaded Ukraine and then withdrew on his own accord, rather than be tempered as collapsing the economy, causing mass poverty and unemployment, and leaving the Russian federation in ruins for decades.