The CEO of Blackberry, Thorsten Heins, said something recently that might be remembered in history as another "there is a market for maybe 5 PCs", or "640k is more memory than anyone would ever need". If the new CEO of Blackberry wants to enter in the history of computers as a famous executive, then saying that tablets will be obsolete in 5 years might one way to do it, although he might not be remembered in a very positive way.
While research firms don't have the best track record, especially when they make predictions over 5 years, all of them are saying right now that the tablet market will be much bigger in 5 years. That's in sharp contrast with what the Blackberry CEO is saying, that tablets will be dead in 5 years.
I think pretty much everyone can see right now that tablets are a growth market, that keeps growing even faster than smartphones right now, in part because they sell in smaller numbers, so there is more room for growth. But you can also see how while a lot of people have gotten tablets, it's still a tiny amount of people compared to all the people in the world, that will most likely get a tablet in the next few years, especially as prices drop and the performance and quality rise.
For tablets to be dead in 5 years, we'd have to already see some other product on the market right now that would make them obsolete. There is no such product on the market today, although the closest one to do that to tablets could be Google Glass, or something like it.
But Google Glass seems like it will be used for different purposes, at least initially, and I don't think it will be able to replace tablets this decade. Even if tens of millions of units will be sold every year by 2018, there would probably be many more tablets being sold per year by then.
So what I really think is going on here, is that Blackberry's CEO is trying to find an excuse to not build a tablet, because he knows that even if he does, it probably won't be successful. It's a game of ecosystems right now, and unfortunately for them BB10 doesn't have a huge ecosystem behind it in phones, let alone in tablets.