Tablets are already dying off, the iPad sales plateau is proof of that.
Why tablets are dying is a more interesting question. Yes, larger phones are intruding on the form factor, but more importantly the general tablet hype has faded away and people have realized that a tablet is not a replacement for a laptop. Vendors love tablets because they're essentially a reset button for software ecosystems: where they couldn't have a walled garden before, now they can.
Touch is a regression in human interface design, and deep down every engineer knows it. To correct that, now we have tablets with keyboards and/or stylii. Jobs swore both of those things would never happen. But I think the reason why iOS gained cut+paste so late was because the only workable soluitons weren't as elegant as he wanted, and he was forced to relent and give the users a necessarily shitty workflow.
Tablets occupy a very narrow market segment that, outside of vertical solutions for business, was doomed from the beginning. Either they'll get replaced by phablets or they'll evolve into netbooks.