Save for the NOS Simplex derailleur I have ratholed away for a project, I don't have in a interest in plastic derailleurs either. But additives have improved the strength and UV resistance in resins a great deal. Injection molding would seem to me to be possibly cheaper to make derailleur parts from than die casting and machining for Walmart and big box store type bikes. The color options plastic would give would appeal to kids. The bikes would still be low quality with short lives, but the derailleurs are usually not the weakest parts of these bikes anyway.