Just FYI I think the sapphire screens are more prone to shattering than the glass ones due to the crystal structure of sapphire. But they're way less susceptible to scratches because of the hardness. These are very different properties!
Scratch resistance is highly important with a device that goes in and out of pockets and can come up against keys and other junk all the time. Also helps if you're good at knocking it with a seat belt buckle, as I am. But nothing will ever be scratchproof or shatterproof. Diamonds can scratch other diamonds, and enough force will break anything. If a meteorite were to come in and hit your Apple Watch, it will shatter the hell out of the whole thing and likely take your arm with it.