90% of these Mac silicon investments would directly benefit their iPhone cash cow—perhaps not this cycle, but certainly in the chips they'll put in future iPhones.

And the remaining 10% would indirectly benefit benefit their iPhone cash cow in the form of keeping people inside the ecosystem.

You have this backwards.

The Mac silicon is inheriting the investments Apple made in the iPhone CPUs. This will continue. The bits which Apple invests to make their existing hardware scale to desktop and high end laptops won't benefit the iPhone much at all. On future generation chips, Apple will spread the development costs over a few more units, but since iPhone + iPad ship several times more units than the Mac, the bulk of the costs will be born by them.

This is the big gotcha. A lot of people see the incremental cost of the CPU as the cost, but the actual cost is:

`(development cost + (units sold * incremental cost) ) / units sold`

But a lot of Macs have higher end Intel CPUs so the per/ unit cost of Intel CPUs is pretty damned high.