I worked on this project to get the Prags over to Medium, so I think I can answer. It's not an either or. They still do ebooks and also print books. For a lot of people, the proper book format is still going to be preferred.
However, there's some things being on the web does that ebooks don't do. The top is that being on Medium makes them Google searchable and that unlocks the reference value of each of these books. You can come in, find your answer, and leave. Most people won't ever buy an ebook just to read one section.
Medium is on its way to become the one stop place for long form content consumption. Bringing Pragmatic Programming onboard is certainly a step in the right direction!
However, there's some things being on the web does that ebooks don't do. The top is that being on Medium makes them Google searchable and that unlocks the reference value of each of these books. You can come in, find your answer, and leave. Most people won't ever buy an ebook just to read one section.