Also, be wary of judging someone by how they choose to spend their free time. Notice how you haven't actually lifted a finger yourself, just judged others for how they lift theirs with zero skin in the game.
> It could be that while a small number of users are helped by his personal attention on the forum, a much larger number of users are left frustrated by the UI.
This is just speculation. Without data to back it up, I can say the reverse is true too.
Since there isn't a large number of users that are posting about how terrible the UI is(on the Calibre forum), it's probably functional enough and people can navigate it.
Buttons look like buttons, not text. It's better than modern UIs. I fear that if he gives in, this and similar small details would change so it looks nice instead of being easy to use.