The problem isn't with PDF itself, but more PDF viewers/creators are such a super-vast-minority behind a single company's product [...] But hopefully GNU PDF can change that
I don't see how. They are behind all the other free software PDF implementations. GhostScript even has a new renderer (mupdf) that is ahead of GNU PDF.
In the same way that Internet Explorer's 98% market share was problematic, and that the web is a better place with competition/awareness of browser choice. As it is now, few consumer know of options other than Reader. (And anybody who's had to use FoxIt for professional-PDF use knows it isn't a working solution for designers.)
FWIW, I think a majority of Mac users don't actually install Adobe Reader, since Preview handles them natively and fantastically for nearly all purposes.
I don't see how. They are behind all the other free software PDF implementations. GhostScript even has a new renderer (mupdf) that is ahead of GNU PDF.