I kinda hope Apple are taking so long because they are trying to really surprise & delight their customers. I've said once before, End of the "all in 1" and introduce a hybrid,
Thunderbolt Display with 4K
&
Completely new Mini that is not so Mini with up-gradable desktop GPU, CPU & RAM.
The advantages for this in Apple's eyes would be,
1) Simplification of manufacturing process = Save lots of $$$
2) User up-gradable parts = customer satisfaction
3) Fix/replace specific parts = Saves more money (not replacing LB, GPU & CPU)
4) Roll out quicker upgrades like GPU & CPU make more money in long run only have to wait on the chips..
5) Lets them move in a more "portable" direction without dissatisfying the customers who want / need desktop performance.
Why replace a whole system when you only want a CPU / GPU bump?
All in a box a little bigger than a Mini that can sit comfortable behind a 27" Thunderbird display.
You could imagine in like Apple making their own Hacintosh Box but obviously freaking good looking.