The mujahideen also had the mountains of Afghanistan. Anyone who is under the illusion that Ukraine can mount a similar insurgency to the afghans has not studied much history.
https://www.washingtontimes.com/news/2022/feb/28/russian-for...
Ukraine's highest peak is 6,700, and the Carpathians are at the very western edge of Ukraine - if resistance merely survives in the Carpathians then the Russians have comprehensively won.