When people say "Eh, it's a living" are they referring to simply being alive? I think people sometimes look too much into some idioms.
In the natural world, you don't actually deserve to be alive and need to compete for it. In societies I would argue it's what the society can put up with. You don't deserve to be alive, you simply are or are not ... or the many thousands of states in between. By default, you are going to need to adapt regardless to keep alive. I can see "living" referring to that adaptation, but then again, I can understand polysemy.