The responses on this thread are pretty interesting.
The iPad Pro is a very different beast than the iPad - at the most basic level the 12.9" display means that it will be significantly less portable than the iPad. I mean think about it - the screen is larger than the one on the 12" MacBook.
The iPad Pro is clearly designed for professional use. Based on the demos given at the announcement, it looks like it is aimed at creatives and professionals who need to work extensively with diagrams.
Of course, everyone is arguing that there isn't any good software for this, but you need to realize that this is a essentially new product category, and time is needed to actually develop the software for the platform. This is a very different approach from Microsoft, which initially decided to go the easy route and enable Desktop apps on a tablet form factor, and of course Windows 8 was the resulting disaster. Here Apple wants developers to develop, from scratch, an optimized, fully featured experience on iOS.
And in order to do that, it needed to increase RAM, enable split-screen, and add a stylus and keyboard. Because it's abundantly clear that any extensive productivity work is going to need all that. Otherwise why does Apple still spend so much time developing the MacBook and MacBook Pro?
But therein lies the key difference - the iPad Pro is designed for a touch-first, iOS first productivity experience, not a hybrid experience. And that's what Apple is trying to pull off here. Will they succeed? Microsoft is already developing a cross-platform SDK for Windows 10. Time will tell. But the iPad Pro is not a replacement for the iPad Air any more than the MacBook Air is a replacement for the MacBook Pro. It can be an alternative, but the focus is different.