Hey I'm guessing by Calibre CLI you mean to do something like manage your book collection, but if all you want to do is convert books, Calibre does come with a CLI called `ebook-convert`. It works with many file formats like epub, pdf, azw3, mobi and more.
It may have been a pain 2001, but it certainly was much better than Windows XP to install around 2006 or 2007, I remember.
I had an IBM R51 (many years old at that point), which used to take 2 hours+ to install Windows XP, and it was always very slow to use. I installed a bunch of different Linuxes over the years and it extended the life of the laptop significantly. Most Linux installers already had a good GUI installer on the Live CD at that point.
Example: `ebook-convert file.epub file.azw3`