If they're going this way, why not just replace the macOS kernel (XNU) with Linux? They'll get so much more.
Because the rest of the system uses a bunch of things that have no drop-in Linux equivalent - SIP, Mach ports, firmlinks, etc.
Those can be emulated with the likes of SELinux, sockets, and bind mounts. It will take a lot of effort and some adaptation, but it could be done.