How big are MacBook and IMac today?
No doubt they could make a better CPU than Core i7 but how much can it cost?
Volume is everything. Something like a server CPU?
Apple doesn't need a new CPU, the one they are already making for the iPhone/iPad basically matches top end AMD/Intel desktop performance category in single thread per SPEC2006. When people say "oh Apple is targeting low power with its cores, no way can it compete with high power x86 designs" they are missing that Apple has only two big cores in the iPhone and AMD/Intel have over 10x as many in their high end CPUs. I don't know what Apple's max wattage per big core is but it is probably something like 4 watts. How many watts per core is the latest Threadripper with 64 cores and a 280 watt TDP? Pretty much the same, and even if it can reach 10 watts per core in turbo that's still a pretty small difference. Maybe Apple could squeeze a little more performance out of their cores in a "turbo mode" if they increased the voltage and power draw to the CPU cores, and ramped the clock up another 10 to 20% but that would just be icing on the cake - they don't need to do this to compete with the highest end x86.
Apple could easily leverage the 'X' version they make for the iPad every other year for their Mac line. It has 4 big cores, which is plenty for most of the Mac line, the only Macs they offer that have more than four cores and might be slower than next year's A14X would be the Mac Pro, iMac Pro and big screen Macbook Pro (which is offered with 6 or 8 cores) They would need to add some blocks to the SoC for things Macs have that iPads don't like HDMI/DP, TB, SATA, etc. but that's only a few sq mm of die size that would be "wasted" on iPads. Not sure how Apple's GPU compares to Intel's integrated GPU but I'm guessing it should be competitive.
They could handle the bigger Macs using the same A14X (or more likely a follow-on A15X or A16X since they aren't going to go ARM across the whole Mac line in a single year - they need to give developers time to port everything before they switch the "Pro" line of Macs) by making it able to act as a "chiplet" ala AMD to build bigger CPUs. Put two A*X together and you have an 8 core CPU (with double the memory channels) for the high end Macbook Pro. Put four together for the iMac Pro. Put eight together for a 32 core monster Mac Pro tower with an insane number of memory channels as the final step to complete the Mac transition to ARM.
Apple simply doesn't need to design a different core, or even a different SoC to go to ARM on the Mac. There were a few milestones on the roadmap they would have wanted to achieve:
1) match x86 performance so ARM native apps would be no slower than x86 native apps
2) retire all support for 32 bit code to avoid doubling the number of libraries they need to support and simplify x86 to ARM translation that will be necessary until developers port their apps
3) retire old APIs to avoiding supporting libraries/APIs that are used by few apps
Those three are checked off:
1) performance levels of their SoC CPU cores nearly match Intel's best in single thread, and since Apple is increasing performance faster than Intel (and TSMC is executing process improvements faster than Intel as well) it is only a matter of time before Apple's single thread performance is better than Intel's best (especially if they can squeeze additional performance out of a "turbo mode" when they have a bit higher per core power budget and active cooling available)
2) & 3) have been addressed by the latest macOS update, which drop support for 32 bit code and obsolete libraries and APIs.
I don't think there are any more pieces that need to be in place to make this happen, so I wouldn't be at all surprised to see this year's 5nm A14X in a couple lower end Macs like the Mini and non-Pro Macbook. Maybe those would be available only to developers at first so they could begin the porting process to produce x64/ARM64 fat binaries, and reach consumers the following year. The big ones using the "chiplets" would follow a year or two later since that would take more testing, and Apple would want most of the important software already available for ARM64 so they need do little or no x86 emulation.
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