Not saying they want to kill Macs, just move them to a new OS so they can share developers with the 10x larger iOS market.

If you have 50% of your OS programmers working on an OS that only 10% of your users use, seems like a waste

But macOS and iOS share enormous amounts of code, if not the majority. The kernel is pretty much the same. APIs usually get support on both systems, with the exception being the UI layer.

Indeed, if they have different APIs on the 2 platforms for the same functionality, you can probably bet that pretty soon one will be ported across to the other and the other obsoleted. Recent examples: PDFs, Bluetooth, Media SDKs (AV Foundation etc).