Who gets to decide what are the inviolable rules of English?

Nobody. One can glean the grammatical rules of a language by observing and recording how users of that language acutally write and speak.

If breaking "the rules" is so heinous, and if new coinages and usages are invariably condemned as heretical, corruptions of the "purity" of the language, why don't all prescriptivists speak the language of Beowulf?

Or Proto-Indo-European or Neanderthalese? Because nobody who speaks Old English holds such views.

Ceci n'est pas un seing.