Haven't we learned in the last 15 years or so that options are bad for users? ;-)
Moreover, watching videos in a foreign language with subtitles in that same language used to be a popular tool for learning languages. Clearly, the proliferation of language skills is a serious danger to the market for AI generated instant translations and must be stopped at all costs.
Also, advertisers get that option, because they're the only users YT cares about.
(You don't want that distracting break in languages between revenue earning productions and embedding media fluff. I guess, in this context, the plug-in is as evil as ad-blockers are.)
In fact, there must be an incentive here for Google to auto-translate a video only when it can sell an ad in the translated language for more than one in the original.
I suppose we should be glad that they've not (yet!) gone that far, or it would become even more of an unpredictable mess from the end user's point of view.
(I can see a bright future where smart TVs come with mandatory, non-optional instant translation… and maybe, if we were allowed to dream, with realtime character replacement and background adaptation to fit the next ad context. It's the logical step forwards from colorized B/W… /s)