I suspect it will eventually happen, probably about the time DRM-free books tend to dominate and the cost of DRM becomes too much to bother with. It will probably be when both major formats (Adobe & Amazon) successors (KF9 & EPUB4?) are getting ready for replacement. At that point, the publishers and vendors can do something like the cell-phone CDMA/GSM sunset with LTE as the replacement. One format/DRM to rule them all! But only after no one cares anymore. Call it 15-20 years from now.
Most people have too short a horizon on questions like this. Big systems move very slowly. CDMA was one of the first cell-phone multiplexing schemes. My 1-year-old phone still uses it. Since all of my phones have been New England Telephone, etc., all of them have been CDMA. That's 15 years.