Apple has been working both openly and behind the scenes to merge the iOS and MacOS platforms. It would not shock me in the least if they pulled a WindowsXP and merged their high end line with their mobile line to have one iOS with MacOS emulation to unite them all. And, with that move, they can merge the platforms into one CUP architecture based on their A series CPUs with maybe Nvidia dGPU units for the high end.
Somewhat, although I'm not sure how deep that goes, and I think most of it is about unifying their software development so that they cut down on redundancy. I do think that unifying their platforms is going to happen, I just don't think they're in a rush to get there.
They're developing their own GPU architecture as well. I have a hunch that a licensing deal is the reason they've stuck with AMD despite Nvidia having much more compelling products. I also think that AMD and Apple's future align a fair amount (APUs/heterogeneous compute, and the multi-chip setup for high performance applications - I could see Apple moving the pro line to basically be add in cards that are just some CPUs with large GPUs and memory, and so if you want more computing power, you just add more cards).
And ti won't happen because it's not worth the hassle. Mac sales are like what? 1/10 of iPhone sales? It's a tiny market in unit and sales/income wise for Apple so ROI to change it all over is not really worth it.
Then the geekbench scores. Can we stop using some in real-life irrelevant artificial benches to compare CPUs on different platforms? It's close to meaningless. Are Apple SOCs great SOCs? Yes. Are they the best? Yes. But if you really think they can compete with a 25W kaby lake, don't be naive. Especially in sustained performance.
Actually I think your reasoning is exactly why it makes sense for them to overhaul (or deprecate) MacOS and effectively unify their platforms. They're wasting resources by developing things for two platforms. With iOS they've got a lean OS that has a lot of modern features integral to its operation, and they're getting hardware that is good enough that they are running out of reasons to keep things separate. And with how things are changing, having software be adaptable to input is going to be a requirement, so even the argument about mouse/keyboard versus touch, etc is dying. And that's going to keep changing as they add more wearable stuff and AR (Apple is developing an AR headset/glasses).
I just think Apple isn't in a rush. They see the reactions to things Microsoft has done, and I think they just are letting things happen gradually. This way they don't upset any of their current product lines. Apple no longer needs to set the world on fire, but they need steady revenue, which gradual development offers.