I completely agree. I talked with the Calibre leader some years ago offering my help with the UX, and he didn't seem to have any interest in simplifying the UI. It's his app, and he wants it to have as much features as possible. It's great for many people but not for the majority I believe.
I stopped using it because I can't stand the UI :(
What do you use instead? Calibre has a rather large amount of features, but they are hard to find :)
Edit: if someone were to publish independent mockups for a new Calibre UX, it could motivate community feedback and/or dev contributions. The ebook market is larger now than a few years ago, given the rise of mobile.
A fork with a sane GUI would kill calibre.