The current policy (as enunciated by Stefan Monnier, who took over as maintainer from RMS a few years ago) is for new packages to be added to the package archive unless there's a very strong argument for their providing core functionality. (Packages that already ship with Emacs are unlikely to be removed from the core, though.)
I've heard it's actually being considered to move org-mode and eshell into packages, provided the release process can be ammended to generate releases which include packages by default.
