Young people are growing up on the mobile phone as their primary computing device, which has fundamentally changed the way they use and think about the internet. Tablets are simply unnecessary for them, because the mobile phone doesn’t offer a degraded internet experience, like it does for adults: it is the internet experience.
I don’t think tablets will ever disappear, but for mass-market use, they’re going to keep getting squeezed from both sides: larger-screened phones and smaller, lighter laptops. The percentage of people whose primary computing device is a tablet may have already peaked.
Over the next few years, I suspect an increasing number of people will choose not to replace old tablets, instead just choosing to use their phones for everything (and computers, if they have and need them). Ironically, this is exactly what tablets did to PCs.