1. Many textbooks are written to be understood, but they vary a lot by field and class level. Generally, I think lower level textbooks best meet Downey's standards.
As you get into what is junior/senior (300-400 level) classes, there is not always a neat textbook available.
2. I disagree with 10 pages per week per course. I think the expectations of what students can read per week are too low. I attended a couple different schools, and one has a reputation of having high expectations of students, and most students tend to rise up to the challenge. I think most professors don't expect enough, and what a degree represents is watered down.
I do feel strongly that busywork and pointless readings should be avoided. Pages per week should not be some sort of metric for learning, but 10 well-written information-rich pages a week per course is not usually going to be a challenge.
Nationally, most students don't even read much of what is assigned, so telling students to not read a book if it has 500 pages won't change the status quo.
3. The idea that writing a textbook is easy is crazy. Even if you ignore the other requirements put upon professors, it is time-consuming to do it right. Even short niche books, think O'Reilly type stuff, take time to produce.