I’m not really convinced.
I read 900+ page books on-screen way more often than I do on the printed page. I’m not convinced that I’m the only one.
There are definitely media outlets that specialize in shorter, shallower content. Some of them are doing quite well. There are other media outlets that specialize in longer, more in-depth content; some of those are doing quite well. A couple organizations are doing a really good job with both forms simultaneously (Buzzfeed, Cracked, Vice – all purveyors of remarkably good journalism these days). I’m sure that some people are transitioning to shorter-form media who are used to longer-form media; that said, the short-form boom peaked several years ago and we’re starting to see a lot more emphasis on long-form, which probably indicates that more people are focusing on longer-form content lately.
One very clear change in terms of journalism & media lately is formalism – and I think this is more widespread and more interesting than the length issue. The best written journalism these days is coming out of organizations that are essentially joke sites (Buzzfeed only fairly recently started pumping out the good journalism, but Cracked has been the home of dick-jokes-with-footnotes ever since O’Brien took over), mirroring the general trends in television journalism (you know, with the best journalism and fact-checking coming out of Daily Show alumni).
The internet changes how media is consumed. But, it changes how media is consumed in very complex ways. Widespread access to high-quality streaming video has simultaneously created a market for extremely short media (youtube shows that average out at 9 minutes; 15 second films) that couldn’t fit into the schedule of a TV channel or a feature film and also long-running, densely-packed serial shows (Mad Men, Breaking Bad, House of Cards) that are best consumed in marathon viewing – it turns out that there was a big market for video that doesn’t correspond to existing notions about ideal length, in the same way that LPs made it possible for prog rockers to make the longer-than-three-minute songs they always wanted. Long before that, the internet made it feasible to distribute extremely long documents with a limited audience – books that would never be printed because the paper would be too expensive, but that now can be distributed for free to anybody who wants to read a six thousand page fanfiction about what would happen if Captain Kirk was a genderbent space hippo; then, ten years later, we got the world wide web, and that encouraged the distribution of extremely short snippets of text densely linked to other extremely short snippets of text in such a way that context can become a part of the content.
I really don’t think that a lack of sustained focus is a modern or recent thing. (To be fair, though – shut off notifications, because notifications can and will cause you to lose focus.) The difference is that it used to take a lot of focus to complain about your lack of focus in a way that reached ten thousand people. Now, all you need to do is write about it and post it somewhere and you have a non-zero likelihood of reaching that scale of audience. (In other words, ADHD is no longer invisible, but some people misinterpret greater visibility for greater prevalence.)