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I found this question particularly interesting because I hadn't thought much about the connection between decreasing attention spans and chapter length, but I do think the two are somewhat related.
Like many things in the writing world, I think the answer to the question largely depends. You see, I don't believe that there's a magic one-size-fits-all chapter length that works in every situation. In a sense, chapters are like paragraphs in that their length depends largely on their content.
Yes, decreasing attention spans are an issue, but I think they're less correlated to the physical length of the chapter and more to the writing itself—most readers, I imagine, would have little problem reading a thirty-page chapter as long as the writing keeps them interested. On the other hand, a five-page chapter could be unbearable if the story is boring and the writing simply doesn't grab the reader.
In my experience, at least, I've found that chapter length is often influenced by the style and tone of the book—some books are filled with fifty+ short 2-10 page chapters, while others are divided into significantly larger (and fewer) sections. Furthermore, I suspect that average chapter length is also probably affected by genre, although I unfortunately wasn't about to find any statistics on that online (although if you find any, please feel free to leave me a link in the comments).
So what does this mean for writers?
While working on your story, I would worry less about chapter length and more about keeping the chapters interesting. Chapter length tends to come somewhat intuitively to writers—we often have a good idea as to when it's time to start a new chapter, and when we get it wrong, there's always editing. If your story calls for longer chapters with many sections within the chapters, then so be it. Short attention spans don't matter as long as you keep the reader interested.
Those are my thoughts, but now I want to hear from you: do you think long chapters are becoming less desirable over time? How do you decide how long to make your chapters?