I think the venue of research/engineering is Tribology, the science and engineering of interacting surfaces in relative motion, where metal still has the upper hand WRT plastics and composites. Basically as soon as you've made a non-metal chain that has a survivable load at the contact surfaces and sufficient overall strength, the only remaining "advantage" left is that you can say "hey, it's not metal". Belt drive bikes are available but are more along the lines of "different" rather than "better". Sure, the potential life of a belt can be longer than the potential life of a chain. But honestly how many people actually see chain life as a limiting factor on a SS/IGH bike? And yes, the belts don't need lube, so if you need to bring your bike on trains, buses, or have to carry it inside your car often you have removed a potential contamination problem. Assuming you had it in the first place. And that you just can't see yourself using a chainguard. A belt would still be able to snag a floppy trouser leg, so that risk is still there. So they address some rather specific problems, and since there aren't any belts that can be opened and closed yet they require special frames. And should your belt fail, or be vandalized, you'll have to get hold of an exact replacement. No cutting to length here. (well, there's probably some leeway depending on dropout/bb design, but nowhere near as generic as the traditional chain.)
