The Speed Metal band Metallica used to say, "Kill 'em all." But in a scholarly treatment of the world's worst wars, from ancient Athens to the Thirty Years war and from Napoleon to the Confederacy, Spengler says that's not necessary. Killing just 30% of the military-aged male population appears to do the trick.
Rather bloody mathematics, that, but his citations are chiefly to solid academic works. I wonder if he hasn't identified something universal in the inflection point. If so, I wonder why that universal would hold in such different times and cultures.
UPDATE: Thinking about this more overnight, I notice the omission of two examples of war of attrition that I'd have expected to see: the Vietnam War, and the Russian war in Afghanistan. Figures for Vietnam are controversial, but the high-end numbers don't support anything like a 30% loss of military aged males: the population of Vietnam actually grew rapidly during the war, from 28 to 45 million. I'll estimate military-aged as roughly 1/3rd of any population, with males being roughly 1/2, so 1/16th is the very rough estimate for military-aged males in a population. The highest number of estimated Vietnamese killed is 3.1 million; 1/6th of 28 million is 4.7 million. But in fact the population will have added nearly an additional 5 million military-aged males during the 20 years of the war, with those deaths being spread out over the whole course of the war and not all happening at once.
Thus I estimate deaths would have needed to be twice what they were and then some to attain the 30%, and that's if the highest estimates of dead are accurate. If the lower estimates are accurate instead, then more than six times as many deaths would have been necessary to attain the figure Spengler describes.
For Afghanistan, the total population is thought to have been around 16 million in 1981-2. 1/6th of that is 2.7 million. The 9 year Soviet war killed -- again, taking the highest estimate -- 1.6 million. So once again, in spite of Soviet brutality, they never approached the figure Spengler describes.
So these two possible counter-examples, in which industrial powers attempted to use attrition as a strategy, both fail to disprove his argument.