I can't answer this question directly, I don't know if it's possible to have a conclusive answer given all the variables and counter-factuals involved.

However, I want to point out that the numbers of Russian casualties attributed to WWII military deaths has often been overstated due to poor understanding of the demographic data.

For the male cohort born in 1923, who would have turned 18 in 1941, about two-thirds of those were dead by the end of WWII. This figure, along with some much higher, is often quoted without context or explanations.

It is potentially very misleading because many more of those deaths are attributable to infant mortality, childhood mortality and political and economic instability within Russia than are directly attributable to the war itself. Of the male babies born in 1923 nearly half had already died before 1941.

Source - Mark Harrison, Professor of Economics at the University of Warwick.

I just wanted to point out that when you read about Russian casualties in WW2 those dead are not necessarily Russian. Soviet Union had a lot of other nationalities and non Russian casualties makes up a significant part of the total Soviet losses - as I recall around 40 %.

The demographic answer, boring as it may be, is that changes in the population stock over time are largely dictated by their natural dynamics.

There has always been a surplus of male mortality rate at all ages, rather stable through the epochs and geography, compared to that of women. For some reason, Nature accounted for that by making babies born male more often.

If you look at the evolution of population over a short period of time, you might think it's made of great ups and downs, and painstaking recovery after disaster. However, over a longer period of time, if you ''zoom out'', you'll notice it's a rather smooth line. When a major demographic event ''happens'', its effects ripple through a few generations and peter out eventually.

I'm not a historian or an aspiring one, but I did major in demography (that feeling when you can finally answer something on this sub) so I can give you a broad idea of what happens when plague hits or world war breaks out. A historian could say better what the case was for Russia, but if I just run numbers from Richard Evans' Third Reich at War, it's likely that Russia recovered swiftly in terms of population stocks. They could also forcibly resettle people which surely made things more simple if a little more oppressive.

I'll edit this later maybe with more, I have old papers somewhere but now I'm too le tired.

Edit 1: gonna name this "edit 1" since I'm pressed for time at work and I know I will have to edit again with my sources, say around 6pm eastern time. It is a universally acknowleged truth however that no matter how busy you are, there is always time to reddit.

When I said "natural dynamics", I hardly meant resources but rather the natural cycles of reproduction and death. War usually does not fall under the umbrella of natural dynamics and its demographic effects are very pronounced although very short-lived. There will be fewer men of a given generation, and therefore they will have less kids who in turn will have less kids (by the fact that they are not as numerous). This is soon offset by the reproductive habits of peace time (because a baby-bust often happens during war and a baby-boom after, the severity of each being variable to circumstance).

Some countries will also adopt natalist policies such as subsidies to families depending on the number of children, paid maternity leave, etc. So even a population deficit is not hard to overcome. France for example has historically feared depopulation. From the day the french authorities started taking demographic data, they compared themselves to the german states and noticed they were far outnumbered. This was the "French school of demography", largely aimed at increasing population through social policies and immigration.

It seemed like common sense that France would suffer from depopulation as it seemed to be involved in every war. However, modern demography tells us there was never really any risk of depopulation, war and disease being offset by the fact that french women had over 10 children at some times.

It's important to remember that the individuals in a population are rational actors and will reproduce more, or less depending on circumstance. During plague or war, less babies. During peace, more babies. This is why it is the natural cycle of life and death that plays the bigger role in determining population numbers and demographic trends. Even large-scale migrations hardly make a dent in the natural flow of things (so much for a migrant "crisis" in europe)

edit 2: A small shopping list of sources I find useful. Excuse the French for a few of them.

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HISTORY:

Richard J.Evans, Third Reich at War. It's just really useful in the sense that it contains a lot of numbers. I used it for an assignment on the battle of Kursk a while ago.

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DEMOGRAPHY [i'm more comfortable using those]:

Ageing and death:

PISON, Gilles, "Le vieillissement démographique, plus rapide au Nord'', Population & Sociétés, No 457, Juin 2009. (examines a simple demographic trend. You can see on the age pyramids that some locales have had disrupting events. They ripple through a few generation but don't shape the pyramid outre mesure)

BONIFAZI, Corrado et STROZZA, Salvatore, ''Cadre conceptuel et collecte de données en matière de migrations internationales'', International Migration Review, 22 Septembre 2004. (No idea what the original language is and I haven't read it in years but it goes over the effects of migration on just about everything)

Eh, I'll leave it there for now. It seems complete enough. At least for everything I can contribute.