Sorry I made you feel like this. There are no shortage of “Calibre but well designed” tools, as long as Calibre has existed. I am just tired of hearing Calibre and other open source apps are badly designed as if that is an objective fact. So I ranted against a general sentiment rather than your arguments in particular and I did not mean to put words in your mouth or offend.

However, when can we accept a design stands the test of time and maybe it is in fact well designed? So many objects that are considered well designed today look that way because they got refined over the time and designed through their goals and constraints. Only in retrospect we call those well designed because they were original and not designed by committee. Can you give benefit of the doubt that maybe Gimp’s, Emacs’ and Calibre’s of the world might be in that category?

As far as I was aware, there definitely is a shortage of "Calibre, but well designed" tools. What are these tools?

There are lots of things that do bits of what Calibre does (Kavita, Librum, KOReader, Koodo, etc), but nothing comes close to replacing it.