I think Apple has painted themselves into a corner with the Apple Watch. Don't get me wrong, I love mine, but it took water resistance, speed and significant battery improvement to move me off my series 0 to a series 3.
Maybe some series 1 owners will move off for a slightly faster, and better battery-life model, but I have a hard time thinking of what would make me upgrade.
If their strategy is incremental updates, they are going to run out of people on older versions.
I can tell you the one thing that would make me buy a new Apple Watch: Extensible Watch Bands.
If Apple allowed 3rd parties to pass data through the band, it would be a game changer. Then you could have real health functions.
Non-invasive glucose monitoring will come, but Apple doesn't want to deal with the FDA, plus, you need the underside of a wrist to do it. Just think about what accessories could be made if Apple opened up a port.
Yeah, I know it won't happen, but short of that I can't see selling my series 3.