Content-based instruction (CBI) is sound and sensible, for textbooks.
In my opinion the textbooks of the school subject of English, or French, or whatsoever language should contain really interesting topics, and not only like rather old-fashioned books, which brought boring conversations in English.
The areas of contents can be very varied: history,
history of the English language,
Catholic religion (in Spain for instance),
stories of real human interest, etc.
It’s a relief things are changing and course books compile texts of varied topics. / Photo from: girl-reading-book www proxystation com