This is because ARM for IPAD is designed for light task for web browsing, sample graphic and causal video editing, all those function can be help by DSP which Apple custom design, so benchmark is good, price can be low however if outside scope, the performance of Apple CPU will be very low, there no problem for IPAD or IPHONE because no people use those for real complex task, the user behavior will highly controlled by Apple in IOS.
In OSX, user want more flexible and full desktop behavior like working for programming, run custom filter in Photoshop....., use for complex sound process, which the Intel CPU is designed for powerful APU with very flexible instructions set for more general computer.
If Apple want use ARM, at least Apple need increase the die side to add more function in the chip, and this will increase the cost.
Change CPU from Intel to ARM will not reduce power usage so much, this is because
Intel and ARM CPU for light task both are very power efficiency just use one to few watt, the external devices such as memory, ssd and display will affect the running time so much.
Also, Intel provide full laptop solution (the cost will share by all PC company), if Apple make their Laptop/Desktop Class ARM CPU,
Apple need huge of human resource for re-design circuit and ARM company will not help so much because they lack of experiments working on this.
I don't thing use of ARM CPU can reduce the cost of MacBook so much, IPAD pro is not cheap device, don't thing ARM can be cheap. The Intel device can be very cheap such as <100 dollar ATOM tablet.
