Actually, I don't really believe this. I do use/consume a lot of plastic, but I find in practice that in the vast majority of cases, the decision to make those items from plastic is an arbitrary one. The market has made decidision on my behalf, and provided me with a range of plastic options, with very few non-plastic options, for an application that has no hard dependency on plastic.
Electronics would be one of the big ones, and can largely be (and often are) made with aluminium* casings. Fittings have similar metallic alternatives. PCBs are something of an exception here: yes, there are aluminium/metallic substrates, but even those are epoxy-insulated.
Most other plastic uses are similar stories: we're provided with a range of exclusively plastic choices by the market, but that doesn't make it necessary on the production end.
Plastic is usually cheaper, yes, but this is a result of a long chain of subsidies, wars, and other market interference. This doesn't reflect true production cost.
* Worth noting, the environmental impact story for aluminium/other metals isn't too great either, but that's a separate topic.