I’d want to actually look at a plastic zipper before thinking it might be better. Note that with the metal zipper in the picture, it’s those tiny teeth (very tiny!) on the back side of the zipper that interlock, not the big ones that naturally draw your attention (those are just there for alignment within the slider). I have a hard time seeing how such small teeth, if made of plastic, could hold up in the long term, so there may be a structural difference there in a case where the teeth are plastic. I’d want to see such a zipper for myself, or a good picture of one.
Also, here, each tooth is individually embedded in the rubber-like material, and it’s that material that failed. What the teeth are actually made of is not relevant. The only way I can see that plastic teeth might be better is IF the teeth are sewn into a fabric backing, besides being embedded in the sealing material as shown here, so that the cloth backing reinforces the sealing material against being torn. If the plastic teeth are just individually embedded in the sealing material fabric as in the picture, I don’t see how making them out of plastic changes anything. Again, without an example to look at so the actual sealing material can be seen, it’s not something anyone can be sure about.