Is plastic an acceptable replacement for wood? In most cases, yes.
Is it a replacement for steel? Certainly not.
Will it ever be? Maybe.
Steel is more durable than plastic. That's all there is to it. It's harder, it's stronger.
Is a polymer frame fine for a pistol? Certainly. I will take no issue with that.
It is fine for a stock for a rifle as well.
Lots of the parts on cars and trucks are plastic. So I don't think strength is a problem with plastic.
Relative to steel and aluminum, plastic does have a strength problem. Parts are made of plastic because it is cheaper, lighter, and softer than steel (this is desirable in some cases, ex. your dashboard). However, in cases where strength is a primary consideration, like the frame of your car, the engine, the axles, the driveshaft, and the transmission, steel or aluminum is used.
It must be noted that the above statement address typical plastics used commonly in firearms and cars, not exotic composite materials.
Carbon fiber reinforced polymers are a bit different and difficult to address. It's hard to put them in the plastics category. Carbon fiber is more similar in structure to asbestos and graphite in structure. However, I will say that carbon fiber is a very strong material, and provides a very good strength to weight ratio. I'm not sure about its durability though, and it definitely is quite expensive. I really don't know of any firearms that have carbon fiber used in them.
It must be noted that while plastic has made headway in replacing wood in way of tool handles that the plastic is often still attached to the same old steel sledge, pick, or axe head.
Speaking of plastic replacing
wood.
