If you're not willing to accept the ludicrously impossible, then 40k is probably not the right universe to be trying to get a handle on
*girlSays the guy who happily discards the setting's most recognisable tagline.
I don't discard it, but I do regard it as a bombastic and overblown marketing tool / propaganda, and can point to sources that prove it to be so.
Edit: Necromunda, like all hives, relies heavily on imported food - its own production is not self sustaining.
Edit edit: Working in a mine doesn't mean you have a terrible life... it means you work in a mine.
The main character of Dead Men Walking is a shift supervisor in a mine, and we see him out at dinner with his girl, he's thinking about proposing and various other "normal" things. Of course it all goes rather pear-shaped when the Necrons wake up, but if they hadn't been asleep down there he would have had a pretty good life.