>The main issue is that none of this is equivalent to the transcript. Nothing of what you propose would make the video accessible to a deaf person. A textbook chapter or the lecture materials are alternatives to the video, it helps making do without the video, but doesn't make accessible the part of video where the lecturer is writing something interesting on the blackboard and explaining something that you can't hear.
That's overstating it. The relevant metric is, if some deaf (or video-hating like me) person wants to learn the material, do they have a useful option? The fact that you can't make that specific video useful isn't relevant. Even pointing to some text that hits the same key points is major step up, and is trivial to do, and yet most refuse to do, just like most refuse to provide the one insight that they're making me watch a three minute video to hear.
For my own teaching, no, I don't write up a "script" beforehand, I prepare content and plan for the topics and key points that I intend to cover in this lecture (and the next one, if we go quicker than expected), but the actual content heavily depends on how the students react, what they are (mis)understanding, what are the questions, etc. I don't consider well-scripted lectures a good thing, the lecturers that I know who are more into "reading off a script" give quite weak lectures IMHO.
Preparing in-depth textual material would seem a waste of effort as students would get the same content in the lecture. I wouldn't use it for other students since I'd anyway never give the same lecture twice - research is progressing fast, so my 2016 content is dated now and this year's content will definitely have to be rewritten (and the course restructured) next year. Admittedly, that's context-dependant - my situation is uncommon, there's a lot of teaching that involves lecturing the same introductory topic for many years off of a classic textbook to multiple larger audiences, and in that context your arguments would definitely apply.
But that's actually all offtopic.
The main issue is that none of this is equivalent to the transcript. Nothing of what you propose would make the video accessible to a deaf person. A textbook chapter or the lecture materials are alternatives to the video, it helps making do without the video, but doesn't make accessible the part of video where the lecturer is writing something interesting on the blackboard and explaining something that you can't hear.