Apple only sold 12 million tablets in the last quarter. Only? That's still a huge number, especially when you look at first time buyer numbers. 40% of US and 70% of Chinese buyers are buying their first iPad. iPad ownership is growing, it's just that buyers are holding on to them for much longer.
Apple hasn't really managed to produce a compelling reason to upgrade. Everything but the original iPad is getting iOS updates and everything but the original iPad is still viable as a tablet device. They don't take the same level of abuse as smartphones so consequently last longer.
So the compelling reason for an upgrade has to come from Apple, in the form of a dramatically improved iPad. Is the iPad Pro that iPad? Given that it might not ever exist that's a big ask.
So what can Apple add to the iPad to make an upgrade a must? Personally I'd like a stylus, a proper active one like the Surface Pro has. There are that many stylii on the market for the iPad I'd guess I'm not the only one.
It probably doesn't need to get any thinner and lighter, and an even higher resolution shouldn't be a priority. I'm not entirely sure that growing the screen, like the iPhone 6, will be a winning strategy.
On the other hand Tim Cook seems quite happy with the way things are going, so he's either comfortable with the way things are progressing or he knows what's coming next is going to blow our socks off.
I'm guessing it's the socks thing.