I agree that a leaner core seems intuitively right. However, the discussion on emacs-devel has been in the other direction as of late https://lists.gnu.org/archive/html/emacs-devel/2020-04/msg01....
> A lot of people have not been contaminated by the entitlement that comes with the "I'm sending a PR, please check it in" generated by the Github culture.
I don't think the person who wrote that has even looked at the pull request review that goes on in high quality emacs packages on GitHub, let alone participated in it. This sort of thing really makes it clear that some of the people on emacs-devel are hopelessly out of alignment with the majority of the people would would willingly contribute to the emacs and emacs lisp package ecosystem.
There are many good reasons not to included packages in core Emacs. A big one is that package authors are working in their own free time and don't want or need to have to prepare an especially bug-free release to coincide with Emacs release cycles. Also they don't want to support users using the old version that is in Emacs.