Commodity resins are used to replace metals, and in structural parts all the time. Much of it comes down to part design, filler selection, etc.
For example, glass-filled polypropylenes are used all the time as metal replacements in vehicles (injection molded), even in some structural applications, depending on the exact demands.
Nylon is a pretty good structural material, especially glass-filled or talc-filled depending on abrasion resistance and rigidity required.
I don't have experience with the very expensive stuff (PEEK, etc) though, as usually that's much more expensive than the metal and doesn't confer many benefits except in pretty high end applications.
For example, glass-filled polypropylenes are used all the time as metal replacements in vehicles (injection molded), even in some structural applications, depending on the exact demands.
Nylon is a pretty good structural material, especially glass-filled or talc-filled depending on abrasion resistance and rigidity required.
I don't have experience with the very expensive stuff (PEEK, etc) though, as usually that's much more expensive than the metal and doesn't confer many benefits except in pretty high end applications.