There's long term dynamics: just as people first prefered a small phone over a large tablet, and now buy near-tablet sized phones (iPhone X is 5.8") that better

There's long term dynamics: just as people first prefered a small phone over a large tablet, and now buy near-tablet sized phones (iPhone X is 5.8") that better match their usage today... watches may well grow in size and features (camera) in the future. Fitness-specific devices (e.g. fitbit) seem more popular than the general-purpose apple watch... at least, so far. BTW non-screen control (eg voice control, buttons on headphones) seem logical for watches, but apple experimented with this already with the iPod shuffle, but removed it in the next version.