Just going by the number of Mac sales, around 4.2 million last quarter & increase of 4% YoY. The iPhone sales went down YoY & I think they've already peaked annually, the decline is less stark atm because more markets, like India, are getting the latest iPhone as opposed to a dated model, also there's growth outside of the traditional Apple strongholds.Whoops I was looking at q1 #s.
I don't understand your line of thinking at all. Maybe some made up numbers would help. How much do you think it costs apple to make a $1500 macbook pro? How much of that do you think goes to intel? How much do you think they would save by making their own CPU? What is the profit margin increase on the average mac?
I don't see any reasonable set of numbers resulting in a profit increase that makes the extreme amount of hardware and software engineering work make sense.
As for how much does Apple pay, I'd expect at least a $100 worth of Intel goes inside each Mac, a lot more for Iris GPU parts & likewise the desktop & Xeon parts. The net (%) margins on Mac might be lower than iPhone or iPad but per unit they make a lot more as well. If I take your number $1.75 billion saved, or just a billion for arguments sake, that's still a good billion saved. The thing is Mac aren't slowing down in sales, they're also getting more expensive at retail, whether due to component costs or more profits for Apple isn't clear atm.
As for hardware engineering, that's an ongoing process, the Ax have come from single to 2+2 core config. There's no reason to think that Apple won;t go full quad or octa core next time, an Ax SoC already has the highest IPC in it's TDP range, possibly even higher than core m. What makes you think that Apple won't go for more software integration ~ http://www.macworld.com/article/3120857/macs/macos-sierra-review.html
A large part is already underway, it's just a one time effort not unlike the last transition from PPC.