I think it is in bad taste and form to upload a KF8 only book.
- Doing so thumbs your nose/beak at all the Kindle owners who do not have a KF8 capable reader/app
- Those who have at least one reader capable of reading KF8 and at least one that is not will be forced to use only the KF8 capable reader
- Even if you upload two versions, one KF8 and one mobi, the two versions cannot be Whispersynced as one but will be treated as two distinct books. So if a person reads the KF8 version on her/his Fire and the mobi version on her/his K3 they will have to maintain two separate sets of bookmarks, notes, and locations read to. That would be a major PITA.
The only exception I can think of would be a book that requires significantly complex formatting that is only available through KF8. I can think of NO
exceptions for novels, short stories, et cetera.
My thinking is that mobi should be offered for at least 2 or 3 years after Amazon stops selling mobi only readers. Amazon is still selling mobi only K3's and DX's. A few years down the road when few mobi only readers are still operating then KF8 only could become the norm.