Great question! Its super simple, it is ONLY the length of the hydrocarbon chain, a better quality natural rubber has really long chains (10,000+ atoms long) that last a LONG time and are VERY stretchy. Synthetic rubber (or plastic) is shorter (1,000 atoms long) and doesn't last as long.

Thats it, it is the exact same "product" just a chain that gets longer and longer and changes its physical properties as it grows.

Noob here, but aren’t the best tires made from natural rubber? Or am I mistaken?

It's more than just the chain length. The most common synthetic rubbers are styrene-butadiene copolymers. Natural rubber is polyisoprene. While it is true that shorter chain synthetic polyisoprene is available, it is a much smaller part of the market than styrene-butadiene.

Does look like both natural and synthetic polyisoprene is at-least somewhat biodegradeable: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC92035/

Genuinely interested here. If natural rubber and plastic are exactly the same thing, then why is natural rubber being advocated as a much more environmentally sound alternative? Would'nt the dust from natural rubber tyres be just as problematic as the plastic is now?

Yes this is exactly my question as well.

Is the dust just as bad for the environment, but natural rubber wears more slowly due to the longer chains?

Or is the dust somehow a less harmful kind of dust?

natural = good; synthetic = bad (◔_◔)