The metadata would be there to make full use of.
It would 'live' in the database/library file.
Sure there are dedicated programs for managing videos, ebooks, documents, images.
They all have their own strong points, but they also have their weaknesses and limitations.
But every kind of document has specific tags that don't exist in audio tags. Right now, images and PDF are associated to audio files in MB and their multiple specific tags don't need to be supported as they would be if MB had to manage them separately.
I own thousands of digital comics and use ComicRack (sadly abandoned for years). I can organize, group, filter by Series, Volume, Number, Title, Published date, Released date, Crossovers, Imprint, Writer, Penciler, Inker, Colorist, Letterer, Characters etc. (all these tags are scraped from the ComicVine site). Then would come the need to find dupes, convert etc., all things we can do with audio files in MB.
Documents (Office etc.) I don't know, I only have a few and I don't need some specialized software. But images, videos, eBooks have a lot of specific tags too. To support the most used digital files would mean support a huge number of new tags and/or custom tags and some new features you'll find even in the most basic specialized managing software.
As for video, managing them with MB without the ability to play them directly would be pretty much useless. Media boxes have their own integrated managing software and you can even install Kodi or Plex if you wish for something else. The only benefit would be on PC to watch videos from your HD/SSD by opening your video player automatically when you click on a video file in MB. I'm not sure how much people still do this.
Still, if Steven is temporarily bored by MB, I suggest a comic-book manager (joking