You can do wood sculpting, but I imagine its more expensive. We used to do pewter miniatures a little, but they're harder to assemble and scratch paint.

Honestly, miniatures are a long form low volume use of plastic and are a drop in the ocean compared to single use packaging, even if the plastic is the same. I believe 40k minis are polystyrene which is the same plastic you see workers crushing down at the end of a grocery fill shift.