I agree that innovation is starting to disappear on smartphones, but the fact of the matter is, the phone market ia totally different from PCs.
If you have a PC for 10 years that suits your needs, power to you. However, PCs are not as versatile as phones.
Phones are used all the time for taking pictures, messaging, playing games, etc. They are on-the-go devices that are always within reach. Unlike computers that you leave at your desk, or take with you to meetings. Your phone never really leaves your side and therefore takes on a lot of wear and tear. There’s a reason carriers replace phones for customers every 2–3 years, because users need more storage, their battery is worn out, and their screen might be cracked. Innovation might slow down, but sales will never fall off as much as PC sakes have, unless we replace smartphones entirely.
Also, the same wear and tear issue applies to watches, it may actually be more applicable. Watches are strapped to your wrist, not protected by a case and pocket, so they get smashed into door frames, get wet all the time, and have to endure being out in the elements. You may end up replacing your watch more frequently than your phone just due to how much wear and tear it takes on the daily.