> Note how nobody ever got the idea to apply syntax highlighting to English or any other natural language

Yes they have? It's a fairly common method of teaching children sentence structure.

Pedagogy aside, take a look at what books looked like before the printing press.

> for experienced readers syntax parsing happens as automatically as it does in natural language without any need of colors.

If you're reading slowly, yes. The colours help you skim-read.

Why do you feel the need to intentionally hobble yourself? The human eyes can take in a spectacular amount of visual information at a glance, using colour, texture, shape and so on to instantly classify and identify things.

Decades of visual design research have proven that colours and shapes improve understanding and you're using a device capable of complex and rich information display, but you want it to look like a book because ..?