And it wouldn't be half as susceptible to fatigue as aluminium is. Probably not, aluminium has been used for a hell of along time as a structural member in Aerospace and the automotive before it ever got used on a bicycle. Correct me if I'm wrong, but I haven't seen plastic very commonly used as an alternative material. Who knows though...
No reason it will be inordinately expensive.....the Truv PC bash's are pretty cheap, certainly not a huge amount more than their aluminium counterparts.
Yesh, forgot about the pedals.....top example. Like you say, it would be fine on something experiencing a relatively small moment. But that torque and thus shear force would be proportionally increased the longer the stem is.