I can consume text at a rate sometimes as high as 26 words a second. I cannot do that with audio. If we had text-to-speech, I would use it for turning audio into text, and consuming the text. Or the author could use it and produce text, which they could then edit. Frequently when talking we do all sorts of things we don't do when writing: repeat ourselves, use funny turns of phrase, search for words, etc. The bandwidth advantage to the consumer of a small amount of work for the producer makes text continue to be valuable.

As far as diagrams go in technical areas, there are some famous pictures in mathematics. These pictures inevitably mean nothing without text. Transmitting abstract ideas, and in particular transmitting subtle variations in how solid something is, doesn't seem compatible with diagrams. Diagrams are good for some concepts, but it's still an art to get good ones. Creating them is expensive, and sometimes they don't work. On the other hand it's hard to beat a good graph for communicating numerical data easily and letting the viewer draw appropriate inferences.