Given the current level of overpopulation on Earth has anyone ever calculated how many people there would be if it wasn't for the wars in , lets say , the last 100 years ?
And after WWII we had the baby boom. But your point is valid in the UK. Without the Great Wars we would have many more OAPs needing medical and nursing care, and pensions.
Many of those young men who didn't come home would be in their 80s and 90s now.
The UK sufered deaths of less than 1% of it's population in WWII and 2% of it's population in WW I
OK the majority of the deaths were military which means young men but as you point out there was a baby boom after the war which might well not have happened without it.
In the context of an event like the black death the effect on the population of wars of the twentieth century are almost nonexistant
I would say you would need to factor in the advancements made in technology during war time to the equation.
Advances in medical treatment of soldiers which then enter the mainstream, computer technology which came as a result of needing lighter weight components for inclusion on Rockets /planes and so.
While not directly related to war many things the prolong life have roots in developments made during the war effort.