History will always be the prevailing reason why plain-text is superior.
Going forward one will always be able to look back and understand (well coded and documented) source code in plain text. Already, however, other formats are showing their age: trying to support old binary formats such as Wordstar, Word-Perfect, Microsoft Word, Corel Draw, and various other programs that lived and died a natural life-cycle. The format wars will also be difficult to support over a long time, already the IV50 video format appears to be lost in time, and various audio encodings may disappear.
We are fortunate that we can still read historical texts. Olde English is a little difficult to comprehend but far simpler than deciphering hieroglyphics. Surely we owe it to our successors to be able to read what we've written?
Arguably the simplicity of the English language is also one of the reasons it is the most dominant around the world: it was easy to code 26 characters into early computers. The poor Chinese were never going to win the early technology race by trying to cram 1,000s of characters into a small number of bits. Now that technology has caught up the Chinese have a chance to do something truly revolutionary (imagine if they wrote their own native operating system!).