>I predict by the end of the year they'll have a style/voice selector for responses.

Style permeates all aspects of writing: word choice, sentence structure, literary devices, pacing, plotting, characterization, dialog, theme, etc. Its a culmination of these things that is reinforced over again every sentence, paragraph, and chapter for potentially hundreds/thousands of pages.

I don't see how a prompt could reliably "tune" all of those different variables over such a large context window without it feeling like a surface level facsimile at best.

Human language isn't verifiable. If I ask for something "dark and gritty", there are endless outputs that are indeed "dark and gritty" yet completely miss the style I had in mind.