Kavita does that, which is what I use (though through the web interface, not using opds on an android app for example), but it would still make browsing just a giant single list.
And I agree, fuzzy has value, which is why I don't want to separate major things like Science Fiction vs Fantasy. But there isn't exactly going to be significant overlap between historical romance novels and an instructional book on erlang, so that book on erlang is just going to get lost in the library.
That's why I separate, it's just too much for a single large directory.