If Apple does that, then they run into the same problems that Intel faces. The reason A-series is so good is that it doesn't have to scale a wide range of performance/frequency points -- they literally have the world's best engineers laser focused on iPad/iPhone chips.
I would hate to see Apple try to do everything only to end up starving the iPhone/iPad class hardware to try to replace a few million Mac processors when Intel ones work just fine.
Apple is clearly capable of running two chip lines (A# and A#X) in parallel (three if you count the Samsung and TSMC A9's separately).
There is no obvious reason they can't be slightly more aggressive, and a plausible future is not hard.
There's a 2 core SoC for iPhone.
There's a 3 core SoC for iPad.
That 3 core base unit gets replicated (along with the GPU) on a die that's, say, 30% larger to form a 6 core A#Z. That die maybe drops some of the SoC stuff that's not relevant to a laptop/desktop (?) or maybe not, maybe it's all relevant --- provide a kickass ISP for image recognition/login, GPS functionality because why not, same with fingerprint, same with some of the sensors --- and easier just to leave on die the others.
This 6 core base unit has connectivity on-board (like HyperTransport) to connect to one (or three?) other other such chips.
Now, with three SoCs, we have a nice set of building blocks for
- iPad (A#X)
- laptop (A#X)
- mini and iMac (A#Z)
- mac Pro (2 and 4 A#Z's)
We also (BTW) have a nice building block for Apple's data warehouse, which might help reduce both the annual payments to Intel and the power costs...
Intel's line is such a goddamn mess in part because they're trying to extract every dollar from the market, in part because of internal politics that keeps certain segments alive, in part because of yield issues.
Apple doesn't care about most of these. They aren't playing market games, so it's OK if the A#Z has some extra functionality on board that the iMac will never use. They appear to have vastly less destructive politics than any other company. And they can handle yield by dumping lesser performing chips in less demanding products (eg dump weaker cores in iPod Touches, or in Apple TVs, or in their future Amazon Echo clone, or in a future Airport Base Station(?). )