The obituary for long reading has been written roughly once a decade since the invention of the telegraph. It keeps being premature.
What the numbers actually show
Completion rates on long features are lower than on short ones, which surprises nobody. What is more interesting is that the readers who do finish return three times more often than those who bounce off shorter pieces.
Depth as a retention strategy
Publications that invested in fewer, longer, better-edited stories have generally outperformed those that scaled volume. The economics of attention reward the thing that is hard to produce.
Which is, in the end, a hopeful finding: the format that is most expensive to fake is the one readers keep coming back for.