I am unsatisfied with their explanation for discontinuing EPUB. "Too costly" is a subjective judgment, and they did not reveal the objective measurements or methods they used to reach it. A pack of gum is costly if you already spent all your money on something else.
With the knowledge of what they do continue to support, I suspect that some of the costs they are paying for maintenance are the result of poor architectural decisions. If all book formats had a common source format, supporting any particular book format would be a matter of maintaining a compiler/converter for it, which could be a large initial cost followed by a lower recurring maintenance cost, shared across all books in the system. However, open sourced, community maintained programs are readily available, along with proprietary commercial offerings with paid support.
I know that the difference between raw web site and EPUB is some OEBPS metadata and a ZIP program. The difference between EPUB and iBook is a conversion program maintained by Apple. The difference between EPUB and Kindle is a conversion program maintained by Amazon. The difference between EPUB and PDF is hairier technically, but conceptually it's just non-reflowable fixed page sizes and fonts, which are theoretically handled by CSS "@media print" directives. The difference between PDF and print depends on the printer, but for self-publishing it's mainly just getting the page margins correct between the digital and print PDF versions.
Testing is a branding issue, not a technical burden. You don't actually have to test Adobe Digital Editions support, if you warn people on download that some EPUB readers are not fully compliant with the EPUB spec, and therefore will not render correctly. It's the IE6 problem all over again. If you support only the readers that are currently popular, there is no incentive for their maintainers to move any closer to a standard that would make maintenance easier for you in the long run. You provide the book, not the reading experience. That is the responsibility of the reader.
Mainly, I'm just pissed that they axed EPUB, but still do iBook and Kindle. That is implicitly supporting closed, proprietary formats (or embraced and extended formats) rather than open standards. We have already been there once.
