Maybe without these wars society becomes complacent, oligarchical and sinful (I mean the USA hasn't bee in too many real wars lately and some say we are in decline). Without the fear of Robespierres guillotine and all the wars of ideology for the next 150 years, maybe the people who ran things in Europe would have never modernized their society.
The US isn't in decline, it's just that Asia is finally becoming a developed continent in the manner Europe is. Historically, like half of the GDP of the world was in India and China and now China is resuming its historical role as a top-tier great power. America has pretty robust population and economic growth, with good indicators all around except for income inequality. If anyone is in decline, it's Russia, Japan, and Great Britain. France and Germany are at least on the rise thanks to diplomatic maneuvering in the EU, but Britain is becoming isolationist and without immigration, will end up like Japan.
But on to stay topic, you really have to think about how economic activity is generated. By people. Even though domestic work usually doesn't have an economic value assigned to it, it is valuable and some pay very well for it, so it's a mistake to assume that young men are the only ones contributing to the economy. That said, losing entire percentage points of your population would hinder economic growth considerably. This is also the age demographic that produces children, too, who are necessary to provide for the preceding generations once they're out of the workforce. Honestly, I'd say that the decline in birth rates and the increase in death rates of the working demographic cut growth rates for the next decade by entire percentage points. This isn't even taking into consideration all of the economic activity that went towards breaking and fixing windows, to borrow a phrase. Total war is an economic disaster the likes of which capital crises like the Great Depression have no hope of reaching the impact of.
My views on how (moderate) population growth is instrumental to economic growth is supported by the
Demographic Transition Model, even though education is a far more important indicator according to the paper
Economic Development in OECD countries during the 20th century. Europe, especially Germany, was on the tail end of that transition, with real nice population pyramids due to expanded access to modern medicine and nutrition. These "population pyramids" of plenty of young supporting the old allow for huge workforces relative to the total population producing unprecedented economic growth and prosperity. Of course, France is the famous exception to the model, with no boom, but a steady growth rate all the way up until today. This made France particularly vulnerable to the huge casualties in WW1, since while countries like Austria-Hungary, Germany, and Bulgaria had similar amounts of their total population die, France had fewer young people as a percentage of their population. This means that a greater share of their workforce and child-bearing population had died relative to their retired and disabled population.
All of this means that belligerent powers of WW1 had not only less
total economic growth and activity post-war, but less economic growth and activity
per capita, compared to a counterfactual "no-war" scenario, especially in France. However, quantifying the actual amounts would be worthy of a PhD.