Metal can be tapped into place without risk of cracking. Plastic can crack. Plalastic probably shows no greater density variation than metal. Hardness may be more important than density. Just because something is dense doesn't mean that it is rigid.
Plastic is a broad term covering a wide range of properties.
Nylon, Delrin, Teflon and a number of other plastics are hard structurally sound materials which will hold their own against many metals and may actually show less long term wear. As for absolute historical accuracy, don't use either. On the other hand, both will provide good functional protection.