lamenting that one can't find it much outside television any more and giving it the credit for the popularity of the Harry Potter series

What? Obviously she's never read much fantasy...or other children's books. I love HP as much as anyone, but it's not in any way revolutionary. I could point to hundreds of fantasy series and tons of children's and young adult series that are multi-part.

Not to mention, as glossing said, there's comics, and now manga is incredibly popular in the US as well. Most popular manga series span years, if not a decade or more, telling stories about the same characters. One arc can take anywhere from several months to a year, especially with the adventure and battle ones.