Old readers will eventually die out, as my Sony e-reader can attest and my old Mobi reader. And bad/old app software can be replaced. There are numerous EPUB3 reading apps out there: Readium variants, Books, Kobo apps, Bluefire Reader, ADE, etc for ios, android, windows, mac, and linux (and not to mention calibre's nice new ereader).
Recommending EPUB2 over EPUB3 just to support old dedicated e-readers that are rapidly dying out is probably not a good choice when new devices and apps support both.
The days of no good free epub3 readers is long gone.
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