Simplicity? A guarantee that the file will be readable in 20 years? Project Gutenberg still treats plain text as the default format for a reason.
The ePub standard is 17 years old, it consists of HTML which is 31 years old and CSS which is 27 years old, packaged in ZIP format which is 34 years old, and all are still in widespread active commercial use and very easy to write parsers for. I think you'll have problems with the physical media you use to store your plain text files before you ever have problems finding software to read ePub file contents.