Yeah, a 12.9" maxiPad would be humongous, but Panasonic and Samsung are already preparing to enter that tablet space (Samsung at 12", Panasonic at 20" inches

).
I think Apple are waiting for an LCD panel of sufficient quality. They can't go to a lower pixel density than the Air, but for now such panels are so new that ramping up to Apple production levels is likely ongoing. Competition will be fierce, though, and I'm not convinced Apple is up to the challenge.
Samsung's entry is a 12 inch, 386 ppi beast with 4096x2160 resolution. If Apple can match that, I'll eat an Apple remote!
Ok so you had to drag me again into one of your screen density debates... I was simply explaining how differences in aspect ratios affected screen area vs. diagonals.
Samsung already released a 12.2" tablet (with a whopping 247 PPI screen that nobody complained about) and in most reviews it's been described as laggy and jittery. But I guess it doesn't matter because they were first!
The "ultra hd" Samsung tablet you linked to is just a prototype straight out of their labs. It hasn't even been announced officially yet. It's just another way for them to give a bragging spec point to those who like to point out that "Apple is behind times".
We don't know how it will perform in real life when it is released (and probably sell in small quantities without being deemed a flop, unlike Apple which has to mass produce them) but looking at how their 247 PPI tablet performed and how they didn't mind releasing it as is I wouldn't be surprised if this new 386 PPI tablet has similar real world performance even with a more powerful CPU/GPU combo.
You seem to be obsessed with proving that a higher screen density has benefits. We get that. I don't remember seeing you mentioning that UI responsiveness is something important, even though it's inversely linked to resolution size and screen density.
I'd rather have a responsive 12" iPad pro at 265 PPI than a 386 PPI tablet with a jittery UI.
I hardly notice the pixels on my 9.7" iPad air 2, and on a 12" screen with a much bigger resolution and screen area I feel they would be even less noticeable, just like the pixels are less noticeable on my iPhone 6 vs. my iPhone 4S even if they have the same PPI, just because they're more screen area and resolution used to draw stuff on screen, and I tend to use it a little further from my eyes.
If Apple releases a 12"+ iPad with the same density as the air 2, it won't be a deal breaker for me.
Yes it would be better if they increased the PPI, but the technology for mass producing a thin, light 4k 12" tablet with a responsive UI and good battery life simply isn't there.