> Has calculus or calculus pedagogy seriously changed in the last 10 years in a way that requires a stream of new calc book editions?
Speaking as a math prof: No. No. NO. It is a ripoff of our students, pure and simple.
When teaching calculus at a previous job we used the eighth edition of this calculus book by Varberg et al., and the statement of Taylor's theorem (one of the major theorems of calculus) was wrong. You figure, eight editions, you could get it right. But evidently I was naive.
However, a couple of universities I have taught at have explicitly told publishers that if they go to a new edition, and stop selling the old one, then the department will move to a different book entirely. I've finally moved up to a tenure track job; I'm going to enjoy this game of hardball :)