Do you need to buy a whole new case when the next version comes out, or will you be able to drop in a new module?
That really indicates that Apple Watch will evolve slowly. Maybe it will be three to five years before there's a new model. It doesn't really need upgrades, because it's mostly a fancy terminal for your iPhone, plus a fitness band and iPod shuffle. And I think that Apple will support it for a much longer time than iPhones and iPads. Ten years from now, today's Apple Watch will work with the latest iPhones and iOS.
But they will introduce new cases and bands every year.
I wouldn't be shocked if they did nothing though.