You are exactly right. However it turns out that even with transcoding into multiple formats, the total upload size is often smaller than original, due to the efficiency of encoding itself.
x264 is quite common, and we do use it (implicitly). I am really looking forward to royalty free H265 adaptations (VP9 et al).
Oh yes I see, I hadn't thought of that!
So should ffmpeg and x264 compile into WebAssembly too?
I hope to join the discussion on Telegram when I've some more time.