The author (Byron) seems to conflate "eloquence" with the Ciceronian use of flowery constructions and lots of subordinate clauses. For me, eloquence is the art of making yourself understood in speech. Elaborate rhetorical embellishments, it seems to me, impede that goal.
I find that I identify as a prescriptivist; at any time, there seems to be a small number of words that are widely used in a sloppy and ignorant way by opinion-formers, and that usage rapidly pushes out more precise usage.
Having said that, I welcome the fluidity of English, and would be horrified at the prospect of an Academy Anglaise with a goal of fossilising the language. I enjoy adjusting my register to my audience. And (my elitism breaks through) I enjoy exercising my knowledge of English vernacular (i.e. I like swearing) and formal registers. And I enjoy the effect it has when I suddenly switch registers - swearing during a mainly formal utterance shocks people. I enjoy demonstrating that I am a native speaker, even if that makes me harder to understand for an increasing minority of people.
I use quite a lot of americanisms; when I was growing up, American slang was hip (I'm from the UK). I get teased for that. I also use a fair bit of "archaic" terminology, partly because it seems more precise to me than the modern gloss.
So I guess I'm not really a prescriptivist; I just have some opinions that die-hard descriptivists seem to find offensive. Lexicographers (Websters, OED) have apparently all lined-up behind a descriptivist banner, and I don't know where one might find an authoritative prescriptivist English dictionary.