That's what happened in every lengthy major war: the highly-trained experienced pre-war army melts, and they are replaced by hastily trained new recruits. Still, it turns out they work just fine, even on the offensive: Romans conquered Carthago, the Soviets crushed Germany, the Entente put the Alliance to its knees, North-Vietnam is now the ruling body, Mao beat Chag Kaï Chek, the Whites lose the Russian Civil War, etc.

Assuming the populations remain unto control from the governments, my informed guess is that the deciding factor between the Ukraine and Russia will be economical, as it had been for all grindfest-wars. And at this game, I wonder what will crumble first: the Russian economy or the Western support to the Ukraine – hopefully the former. In any case, victor or vanquished, the Ukraine will end up gutted by what is probably a good contender for the stupidest war of the 21st century.