I'm not too worried about MacOS going in the Pi. Mine, to be fair, is doing very nicely with Raspberry Pi OS (=Debian Buster with Pi "refinements"). Very convenient on Pi when distributed as a binary image. What struck me is that it only takes one nerd to hack into Arm Macs, and the world of Macs is open to Linux. The Arm version of windows sucks, afaict. I've had a lazy eye open for hardware becaude whenever this dies, I'm going to buy a desktop for the long term, and swap components, instead of laptops. Hopefully I won't have to upgrade. If I do it right once, I should be sorted for the foreseeable.

I'll be interested in how this 'Armmac' thing develops, and how linux goes on it. Of course, I don't even know if they're doing a desktop.

With Apple building both the CPU and OS, I expect that they will be more tightly coupled than ever. Apple's goal has always been lock-in. People buy Apple products because they like Apple's ecosystem.

In the x86 world, we take for granted the ability to mix and match CPUs and OSes. This capability does not exist in the ARM world. Apple is not a likely company to change the status quo.
Ed