This is obviously an area where the shifting models of software in general and emacs in particular have changed over time (by a rough count, non-Gnu Elpa is roughly the 8th-10th "emacs lisp archive" I've personally used), and there are clear experiments in play right now. As a practical matter, how would people feel about a sort of autoload variation, included in emacs (30) core, that points users at relevant packages in [non-]GNU Elpa, which would be overridden by code included in an actual package? The idea is to combine a thin pointer with a potential indirection point that allow users (who had not already chosen) to choose between alternatives such as clojure-mode and clojure-ts-mode.
Hope that helps,
~Chad