I just finished watching Stand by Me ... if they wrote out all the script, I'd still be reading while the move had already finished. So it has to do probably with the amount that youu can visually read and yet still be able to see most of what's going on in the scenes.
False excuse. You can translate everything in English when you watch French movies or vice-versa. It works for most European languages. In Japanese it's still easier as Kanji don't take much space and are quicker to read than our alphabet. What's more, in my examples, most of the time it was short sentences with long pauses between dialogues.

Societal structure has remained basically unchanged for centuries on end. Politicians get old and move, Beaurucrates still rule and will rule.
That's quite incredible indeed. However language is not bound to bureaucracy or politics. As you say it yourself, language evolve with society and there has been a lot of change (since Meiji, then then the end of WWII). It amazes me that Japanese language (and society) had changed so little from Nara jidai to the end of Edo jidai or even know. When the Tale of Genji was written, the English language didn't exist (check the history of English here) yet. Shakespeare is difficult to understand nowadays and he wrote during Edo Jidai when Japanese people spoke remarkably like old people still do now. Well, Spanish and Italian haven't changed much for the last 500 years either, but English or French (and German, because of the dialects) have and still do. Japanese has not learnt to create new words by itself, so, like many other languages, it becomes invaded by foreign words. Chinese or Finnish still care to invent new words for almost everything like TV or elevator. But when I see how Japanese people ALWAYS talk about the English language (on every TV programmes,in songs, between friends, at work, or study it during their free time), I really wonder what's the future of Japanese language. I have heard many people saying that they'd prefer to have English as a mother-tongue and insist on using every English words they can in Japanese even if older generations can't understand them. Everytime I sit in a MOS burger (or Mcdonalds), family restaurant or cafe, I see people studying English (with a book or a teacher). That's a national obsession. I young Japanese (between 18 and 30) out of 5 seems to have studied a few months either in the UK, the US, Canada or Australia. Where else than in Japan can you see so many English (conversation) schools/m2 ? Nowhere ; English has become a second language in Japan, eventhough few people are really fluent like in Hong Kong or Singapore (but these had English as first official language in addition of being British colonies).