Just wait until you can only stream books, not download them, with random words replaced with synonyms using an algorithm that lets them track down who the originator of any scanned copies is.

That might sound ridiculous, but streaming-only to prevent perfect copies and hiding purchaser identifiers in the data are both DRM techniques that have been explored in other media already. There's no limit to how anti-consumer publishers can get when they think there's slightly more money to be had.

There's no impossible because if you can see it, it can be captured and digitized, but there is a level of complication that can make it unreasonable. They could make it unreasonable to crack the drm outright and require you to screenshot/OCR it. Then they can limit the OS to make to difficult to automate capture.

Bottom line, they're just kicking payers off their network when it's easier to pirate it than to buy it through their service.