Apple now benefits from being on the x86 bandwagon - by the time it launches a new computer with a new Intel processor, there are already people checking what the new parts can do and optimizing software for them. If Apple is the only company that uses their own processors, they need to bring all that optimization work in-house and manage it in a way it stays competitive with the world outside.
The final point is that I'm not sure how much cheaper for them it'd be. Of course they could get the chips at cost, but they'd need to bankroll their development and all the fine tuning that goes with it.
I would guess that ARM Macs, if not some iPad+Mac hybrid, might come one year after they finally release the UIKit-on-macOS API that they showed off at WWDC 2018.
I'm curious as to how they might solve x86 compatibility.
You know they've already figured it out and it's down to a business decision not a technical one.
I could even imagine they've gotten Parallels involved - an ARM prototype perhaps?
Didn't Microsoft already demo how to do this, where they emulate x86 on ARM? Then again, Intel apparently isn't too happy about that.
What is the relation between 7nm and storage capacity? Is the nand 7nm?
In other words, going to 7nm makes it easier to route wide busses around the chip. You could do the same thing on a larger process, but it's less space-efficient.