> Would anyone be able to access ebooks in 50 years from now?
Access as in the original manuscript of the book is still around somewhere in a readable format so new ebooks can be published in whatever format is in demand then? I certainly hope so.
But should ebooks sold now be readable by anyone in 50 years? I don't think many people worry about that, nor should they. Archiving should be for persistence, so done in some format we can be absolutely sure we can read (such as plain text).
Distribution should be for convenience, i.e. likely whatever proprietary format users want.