I think series' can
be a crutch for both authors and readers. But I don't think it's inherent in the concept of "the series" by any means. I do believe there's a lot of crutch-like stuff out there... but like anything, it depends entirely on the author and/or the reader. It's only a crutch if you stick it under your arm and use it to support something that can't walk on its own. It could just be comfort, too: it's not like literary growth is mandated
for authors and readers or anything.