I keep writing this here and there, so I'll reiterate it here.
When the Soviet war in Afghanistan ended, some 70 000 absolutely mental soldiers returned straight from the front, and there were another 400 000 or so that had been rotated away from frontline duty and were a bit less of lunatics but doing bad all the same. Those 70 000 + 400 000 were too much for the Soviet Union of 300 million people.
One of the most important causes for Soviet Union's collapse were those Afghan veterans who were accustomed to extremely violent way of doing things. The crazy years of 1990's and the famously violent Russian mafia were a result of those 500-ish thousand madmen having been freed to roam the Soviet Union and later the Russia. All that instability eventually led to the total economical collapse of 1998.
So... Now there are some 700 000 soldiers more or less on the front, and another 700 000 doing other military duties. Those 700 000 + 700 000 will have quite an effect on the Russia of 140 million people. Once the war ends, ten times as many lunatics will return to the Russia of only 140 million as returned to USSR of 300 million. That will be absolute carnage and the 1990's will look like a walk in a park compared to what's coming up.
This is already unavoidable, but if the Russia is victorious, it can still avoid being ripped completely apart by that carnage. That's the main reason the Russia cannot end the war. It will wage the war ad infinitum, unless made physically unable to continue. And if they some day cannot get any more soldiers, then that'll finally the physical barrier they've been looking for.