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

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.