https://www.bbc.com/news/world-europe-63102220

Russia can't man its own line. Even with it's call-up and forced participation, it doesn't have the numbers to hold the NE and Kherson. It needs Kherson which provides all the fresh water for Crimea. Russia has had to shift forces out of the NE to stop the Ukrainian offensive.

What is open to Ukraine is a LOT of land to all the way to the Black Sea between Kherson and Mariupol.

Russia's population has been in decline for decades. They can't sustain this long term without help.