Exactly this.
I understand that some people like a whole book management suite. Particularly people who don't use much tech, so it's just their computer + their Kindle + their phone.
But others need something task-oriented. I don't just mean whiny software devs like myself, but students, researchers and - in general - people juggling more devices and document types. Such a user needs a bullshit-free way to view an .epub by double-clicking on it straight in their Dropbox folder. A "send to Kindle" under a right-click menu. A "Kindle sync" that works by having it mount itself as removable storage, so that you can simply copy files over, the same way you put your music on your Android phone.
(Hell, maybe even a dedicated app would be good - one that looks like an orthodox file manager[0], showing you your files on one side, Kindle on the other, and helpfully detecting when you upload the same file twice. But that app would be orthogonal to the concept of opening a book, and to the concept of managing a book library. And I'd still want the right-click context menu integration. Hell, Windows actually had a place for it since time immemorial - the "Send To" section of the context menu!)
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[0] - https://en.wikipedia.org/wiki/File_manager#Orthodox_file_man... - i.e. Norton Commander, Total Commander and friends.