That is a very big possibility. Personally, I think the problem with smart watches and fitness devices is that while they are useful, most people don't actually need the features they offer. For a smartwatch to be useful, you already have to have a phone along side it. But everything you can do on your smart watch can already be done by your phones. That is, apart from the heart rate and movement tracking, which I'm sure only a small number of people are actually interested in.
Everybody used to carry around paper agendas and briefcases with their important files. Now you can completely replace that entire thing with your smartphone. Rarely do I see people carrying around briefcases anymore. Even laptops seem to be losing their purpose as many people just plug them into full sized peripherals and use them as a stationary computer at home or work. In 5 years, you'll probably see that becoming common with a phone using something like Continuum. A tablet could simply be a dumb screen that you dock your phone to.
Looking at things like the Surface book, it's definitely possible to even give your phone extra processing and memory capability when you plug it into the dock. All your important stuff can still come with you, but if you want to do some photoshop work or other intensive work, you're going to need more resources than you can get out of a phone, at least in the near future.