Powerful copyright lobbyists presently circle the globe advocating ever longer terms of copyright protection . . .
This is overheated, and the footnote -- citing another secondary article -- doesn't strongly support it. I don't believe that the US or UK copyright length is in danger of being further extended. There allegedly is a little pressure on Canada (see home page at www.gutenberg.ca
) because of ongoing negotiations over the Trans-Pacific Partnertship. But, in the internet age, extending copyright is quite unpopular. Maybe the author of the OP article would say that only will be true if people like him keep on opposing it.
I do believe that eBooks (even more than print books) published before the US copyright cut-off of January 1, 1923 are more widely available than those published in the first several decades after. It's not just best-sellers that become unavailable after the cutoff, but also many Pulitzer prize winners. This indicates to me that US copyright is currently too long.
P.S. For example, there are post-1922 works of Winston S. Churchill unavailable as eBooks, and also out of print. One is The World Crisis
(1923-31, 6 volumes). Every book Churchill published before 1923 is at gutenberg.com.