For many years after Apple introdued the iPad, sales were increasing at a rate that eclipsed even some of the best selling smartphones during their early days. It was thought that the PC market would die of a quick death after the failure of the netbooks and there was no answer from the PC guys for tablets like the Galaxy. Microsoft's own misguided attempt with the Windows RT versions seemed to have put to bed the dominance of the Android and iOS in the tablet market. Windows was in trouble as PC sales slowed.
Well, not so fast. The PC market did continue to decline for years but lately has shown some resilience and growth in some market even as the general world-wide sale has dropped. It was mainly due to Asus (Gartner).
However, it doesn't seem like people who stopped buying PCs are replacing them with tablets either. Hence, this is likely why the smartphone market continue to experience robust sales with annual upgrades.
Now, the question is whether the tablet market is experiencing a netbook effect, albeit at a much slower place and likely won't go away entirely. Tablets today a much cheaper than when they first came out and with better mobile and touch experiences. And they are generally used for consuming media. So, it is not as if upgrading tablets annually or even every 2-3 years is absolutely necessary.
One could look it it this way: either tablets are built to last longer than most companies expect or that there generally has been a lack of innovation in that market. For instance, there is no reason why tablets with bigger form factors should not have just as good as cameras as their smartphone counter parts. Go ahead, just charge more for it, Apple and Samsung. And while Apple spent the last couple of years playing catch up with the pencil support with the iPad Pro, there has not been any notable innovation in the Android market either. If anything, it's the integration of Google Play into Chrome OS that is most exciting but that had been anticipated for years. (Google)
So, there is still a chance that the tablet market can return to some growth but expericene has shown that it'll be overshadowed by smartphones and possibly gears that support augmented reality or virtual reality (unless tablet markers find a way to support AR/VR) and a PC market that just won't go away. Growth will could come but slowly and the prominence of tablets of yesteryear will never return. But hey, what company does not want to sell a few tens of millions of tablet each year?