[Quoth]

The desktop app for Sony obsoleted by Calibre.
Then Calibre was better than Kindle PC for a Kindle.
Then better than the Kobo program.

Almost all ebooks are read on phones, tablets and ereaders.

A Linux Kobo program isn't needed. I was using ebooks and ereaders on XP, then Windows 7, but with Calibre. I've run the Sony & Kindle one only a couple of times on Windows and don't miss them. Linux Mint with Mate full time since January 2007. I'd not use a Kobo desktop program.

Calibre manages ebooks for my Sony, Kindle, Kobo, Phone & Tablet Apps and Android eink. Also lets me create ebooks from docx (exported from LO Writer), "fix" ebooks bought or PD downloaded.

Desktop programs for one brand of ereader are pointless. Also there is Okular as well as Calibre viewer and other programs to read ebooks on Linux. Sigil as well as Calibre to edit an ebook.

I have the Kobo app on Android phone, Tablet and eink, but I use Lithium, Pocketbook, KOReader (and a couple of other apps on a really ancient Android phone. I don't actually use the Amazon, Kobo or Google ereader apps. The only apps for a particular source I use on Android are for the local Irish Public Library, Borrowbox for ebooks & Audiobooks, and Libby for periodicals and comics, though they are changing to something else.