I'm inclined to agree that the non DRM'd formats are more likely to last long term, than proprietary formats (like most current ebooks). Of course, you might still be able to use existing software to access those formats then, but you might also have to lug an aging device about to run that software on, as well (not likely).
That's why I see DRM'd ebooks as a very short term purchase, which doesn't satisfy (I buy books for life). That's also why I buy lit format books, and keep that handy little utility around, so I _will_ be able to read the books I've paid for in 10 years time.
Craig.