system
Given the following post, respond as if you're commenting on Reddit:
human
[40k] What happens to Space Marine trainees that don't make the cut?
gpt
Space Marine chapters still need an untold number of hands to help them in their ever vigilant crusade against the enemies of mankind, despite their infinite combat prowess, a thousand holy battle brothers could not pilot small fleets of spacecraft that number in the tens to hundreds of kilometres. 

They also need people to generally help around the base, farm, move supplies. The Astartes are entirely dedicated to War, everything else is done by serf/acolytes/failed recruits.