It's way easier for someone to hack up an acceptable new UI for org-mode (whatever that looks like) using the existing underlying Emacs org-mode code/runtime th

It's way easier for someone to hack up an acceptable new UI for org-mode (whatever that looks like) using the existing underlying Emacs org-mode code/runtime than it is to hack up a new UI and completely replace all the Emacs code & org-mode. You can run Emacs headless, it has C extension support now, your Mac app could start it headless and communicate with it over a socket or as a network process. Multiple Emacs modes do this to make Emacs itself talk to other programs (e.g. mu4e <-> mu, gud <-> gdb), there's no reason you can't do it the other way around.