John Prins wrote:
Additionally, the guardsman is more likely to die of old age - far earlier than the Marine.
Not just die of old age, but a guardsman's combat efficiency will drop sharply past the age of 40, so you get at best a good 25 years (assuming recruitment at 15), though anybody surviving 25 years in the guard is a canny soldier. OTOH
, a Space Marine doesn't decline much with age at all, and even if they do it's after centuries of warfare.
Most Guardsmen won't live to see 40, unless they are recruited at that age. The average life expectancy of a replacement in the combat zone is fifteen hours. That's why IG
Veterans are so highly valued, organized into their own units, and get special equipment from the Munitorum (though many reject the nicer carapace armor and hellguns for standard issue IG
gear). They beat the odds and have years of combat experience, without the fancy gear and top-flite training of the Militarum Tempestus. They are also the few Guardsmen that tend to get away with killing Commisars/officers, threatening Inquisitors, and being less than worshipful of Astartes.
The truth about the Astartes is that they do tend to die in droves, even with their better equipment. Especially against serious opponents, like the Necrons or Tyranids. And in the age of the Cicatrix Maledictum, they were being hard pressed. The threat level in the Imperium after Cadia 's fall was enough that Guilliman activated the Primaris project to meet the challenge. That's one indication that the old school Astartes are no longer the absolute cream of the crop.