I think the quiet time is because it is this certain time of the year where people spend a lot of time with their families ;-).
About the question: good answers already!
I only like to add that Japan can very easily rid Russia of those 20 Siberians and even does not need a lot of air to do so. 2STR + 1TAC + 1FTR. The rest can still go down to get dirty on China + UK.
What Russia achieves against Japan is that a J3 crush against India is unlikely if Japan minces the Siberians. However, Japan could also go defensive against the Siberians and still go crushing China + India without a sweat.
So in short Russia achieves nothing of any real value against Japan. It is very easy for Japan to modify its plans a little, so that J1/J2/J3/J4 are all still viable, solid strategies.
Against Germany however, I’d call the effects crucial.
With air from the UK, Moscow should hold GE6 and GE7, even with max investments from Germany to crush it. RU7 then injects 20 extra units in Moscow to make it impossible for Germany to grab it for another 3 turns at least, but indefinately if the allies have built up a massive invasion force in Western Europe. With the latter in place, Germany cannot take Moscow without allowing the allies to invade Europe in force…
If Moscow Always falls GE6/GE7 in your games, you should work on its defense! Else it is indeed no use to try to retreat the Siberians to Moscow. IMHO, the only time it is certain that Moscow falls is when the allies focus a LOT on Japan and basically ignore Germany for the first ~6 turns of the game (without a threat in the west Moscow is indeed doomed). And even then I think Russia is better of harassing Japan from Buryatia than sacrificing the Siberians all at once in Amur.