It's much rather Russia cannot AFFORD to lose, but not that they cannot lose, because they are going to lose in Ukraine. In fact, as I and many had already mentioned here and analyst worldwide said they had ALREAD LOST in term of strategic position, because in one stroke, they themselves had relegated to pariah state of the world, and now it pretty much depends on China to keep the government from malfunctioning. Border with NATO country increases instead of decreases, and then they got stuck with an angry neighbor have enough weapon that can take them down and a collective west that are more than happy to keep supplying them weapon to screw with the Russia. What Russia get in return? the "annexed" state?
Annexing those state is an act of desperation for Russia, that's literally the last draw because they would have to put their money to their mouth and really defend with everything they have when those annexed territories eventually get attacked. Nobody, not the west, not the Ukrainian is going to even entertain the fact that those state are now "Russian Territories". Which mean if this is the case, there are only 2 options left for Russia, they either call for a general mobilisation, or use nuke. To do either of that is basically Putin's political and probably literal suicide.
You asked me before what can the Russian middle class do when they are mobilised? Well, have you seen the state of affair in Russia lately? And that's not even them being mobilising, now imagine when Putin come down with a news that they are going to use nuke and bring them all down with him because of this war, or literally bring them to the war by drafting each and every one of them. What would you think will happen?