Phones and tablets are very close to the point that desktop/laptops got to a few years back where unless there is some major disruptive technology introduced (eg. batteries with 100%+ efficiency over current ones) on top of them the one you bought this year is pretty much good enough to keep for the next many years, until it breaks somehow.

There will still be a market of ultra-fashion-conscious buyers who really need this years model, just like there are people who lease brand new cars every year, but that market isn't big enough, IMO, to sustain the growth that Apple, Samsung, etc have enjoyed thus far.