You're talking about appearance, when Calibre's issue is almost entirely UX design. It's actively user-hostile software that is on a fundamental level not designed to do the things most users want to do.

Step a new user through setting up Calibre to do something as simple as syncing to a e-reader with the functionality a user would normally expect, like displaying/syncing their reading progress.

Or just watch Goyal tell a new user "you can't do that, use the builtin reader if you want reading progress synced, fuck off" and then a bunch of other folks get into the weeds describing the mess of plugins and shit you need to do it.

https://www.mobileread.com/forums/showthread.php?t=296205

I managed to get my e-reader to sync reading position to Calibre but it required a lot of frustrating work trying to follow tutorials on mobiread...creating custom fields and code snippets and praying that the whole thing won't break some time in the future.

It's probably the single most commonly wanted feature for a user looking at an e-book library management tool. That and syncing to their e-reader's shelves.

God help you if you want to use Goodreads...