S.L.A. Marshall was a fraud - he had zero combat time and he made up a lot of his data.
Here’s what I know: there are three kinds of folks in combat - Killers, Fillers, and Fodder.
In a good infantry outfit, about 20% are Killers. They are experienced or just talented and they are steady and they aim. They are the backbone of the fight.
Another 60% are Fillers - or as we used to call them, “noisemakers”. They put out a lot of fire and most of the rounds go in the right direction. Probably good not to have the new guys behind you though. Eventually, if they survive the first few weeks, they usually gain the skills to become Killers.
The last 20% are Fodder. No talent. They’re the ones that stand on skylines, smoke while on Listening Post, open gates, pick up stuff they’ve been told not to, daydream when on patrol. Sometimes their lack of talent gets the others killed.
Even the worst of them though, are worth more than all the rest of their fellow citizens because they’re there, risking everything they have.
What you say is pretty much in keeping with what my dad said. He was a WWII vet, European theater. He said that new guys usually got tagged in their first 72 hours. If they made it past that, they usually made it all the way.