So if we go for Guile-Emacs, we'll be stuck with Guile, i.e. we'd
have (old and new) packages that use Elisp, new packages that use
Scheme, maybe yet other new packages that use, say, Javascript (or some
other language support by Guile). That would make the work of Emacs
(and GNU ELPA) maintenance harder.
So don't accept packages into the Emacs core that don't meet their standards? It's okay to have project standards but limiting how other people use your tool seems terribly misguided for a GNU project.