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.
Anytime a UI is updated, you're going to completely modify the behavior or of the people that are using it. So any change someone else is making that isn't as closely tied with the product as he is, is probably going to be suboptimal. Not only from a user experience perspective, but also from his ability to answer questions on the forum as people are asking him questions about how to achieve certain things.
This means that he'll HAVE to be closely tied with a redesign. Which is probably not where he wants to be spending a significant amount of his time.
There's also the added complexity of legacy users that are just book people, that are quite used to the design as it is, so any migration over the new one, no matter how gradual, is probably going to make the product harder for them to use.
I'm not saying I agree or disagree with him, but his reasoning is valid. This isn't a project where you just hand off to someone completely new and let them do what they want. That's how you end up with a broken product.