I don't think I misunderstand the issue.
> The Emacs maintainers would not have to maintain Magit
That's not true. They would have to maintain it: deal with bug reports, deal with breakage due to upstream API changes in Emacs core. They wouldn't do feature development.
I used to contribute code to org-mode, so I know how this works. The magit team will make periodic releases, at which point the magit code will be copied over into the emacs code base. From that point on, the emacs maintainers are on the hook for any bugs or other problems occurring with the version of magit that they have included into Emacs. They'll have to deal with/triage magit bug reports received via the Emacs bug tracker.