Caps, Closures, Containers, Automotive, Medical, Appliances are the biggest winners when it comes to Polymers. The pro's of plastics come into play when trying to minimize weight and corrosion. My job is in the plastic industry. Mainly for Injection Molding. I am pretty certain that everyone on POAL has used at least one of the components I have worked on in my past. I have seen a working engine with the block molded in plastic. However the cylinder walls were metal. Much of the medical industry drives the plastic industry and a multitude of procedures are only possible because of plastic. I have personally built over 100 molds in my career. Most were single cavity molds in the developmental stage of a product. I often wonder about the misinformed SJW that would find themselves in the emergency room and say "I don't want any plastic devices to be used on me". That is when the staff will make them aware that they are about to die. One of the biggest problems with plastic is that some designers like to use the least expensive polymer that they can. Polymer choice is a large portion of manufacturing a successful component. Polypropylene and Polyethylene are at the low end of cost. Currently around $1/lb. and Polycarbonate around $3/lb. BTW those prices are when you buy a large quantity of material (Railroad Car Full). A more expensive material called PEEK is $400/lb. PEEK is a high heat injection moldable material that is sometime classified as a Thermoset. Thermoset materials are not recyclable. The only product that comes to mind that was better in metal than plastic, is a SLINKY.