I do agree that a “vegan” Hermès band is cool.
Hermès silk vs nylon though? Not even close.
Nylon is what they make low grade rope out of. Nylon is a byproduct of oil. While oil is “vegan” it surprisingly goes against apple’s whole environment initiative.
Nylon is used in a million things from cheap to exorbitant, and the amount of crude byproducts or natural gas (nobody actually uses raw crude oil to make plastic) which go into a few ounces of watchband pales in comparison to the environmental impact of actual leather.
That said, these are tiny products. Yes, the carbon footprint of leather is three times a similar amount of plastic but they're both such small amounts that it need not, in my opinion, be a primary concern. "A pair of leather boots affects the climate about three times as much as its plastic or pleather cousins. However, since the emissions associated with a leather pair are about equal to those from burning a gallon of gasoline..."
If you feel like making it a criteria for watch bands, the nylon one is the most environmentally conscious alternative.
Aesthetically, like most man-made products, nylon can be anything from cheap to beautiful. Prada makes beautiful nylon bags. Hermes makes beautiful nylon jackets and has for a very long time. Why not hold off until you've held and worn one before saying that you know better than their designers?