If we have PDF support (which is probably going to improve in the future), epub support is a likely feature as well.
Obsidian is still in beta, so of they got other things to worry about, but I wouldn’t rule out the epub possibility so easily. Its not something I see as a high priority, but if announced, it sure wouldn’t make me raise an eyebrow like an mp3/mp4 addition would. Its not even in the same ballpark.
However, your point is noted that Obsidian wouldn’t be do good to just add every little thing. I just don’t see epub support in the distant future as a necessarily pointless addition.