Well, without any offensive threat, its not as if you are holding down a ton of Japanese assets; and if you stay near the coast, Japan can use its air-lightsaber to annihilate you. So, best case scenario you hold some of your land but the troops are paralyzed over there.
Then, if Germany is focused, they take Moscow, so you didn’t really do anything but preserve some extra $$ for Germany to take for their war-effort.
It sure ISNT a good idea to lose all that eastern land, most good Axis strategies have some plan to suffocate Russia’s income and the best approach includes trimming USSR east and also hacking them apart from the china-side once japan has run through all those troops. The really good money is right behind Moscow, but if all 3 axis are dogpiling Russia, this backdoor is left open for Japan to take it all, there isn’t much you can do as USSR if they stay dedicated to this plan because you cant both counterattack and protect the capital. If Japan has other goals, that $8-$10 in eastern income for 4 turns can make a huge difference in Russia survival.
So you can stand and fight, but its a stalemate at best and you are going to still lose your income and possibly get surrounded without threatening japan much…and if you lose your capital, it was all a sideshow anyways…
Or most of the guys can run for Moscow, where they might make the difference and buy you some time for Moscow to survive…in a game where the Axis’ only big plan is to kill Russia, if they fail to take it by turn 7-8, that’s the point in the game when the Western Allies can exert some real pressure on the Axis and start the life-support.
If Germany takes Russia’s $$, you’re in deep crap because not only is Moscow not even a distraction anymore, but Germany has $100 to push the Western Allies back that next turn…after that they have little chance of defeating Germany…
TL;DR=protect your capitol at all costs…