Your definition of the Kindle as a "sad, sad reader" appears to be entirely based on the fact that it only supports it's own bespoke format.
And therefore adds the burden of converting to a second format -- something which literally costs nothing but disk space.
I will ignore your remark about OPDS, as I am not aware of any ereader that supports OPDS. I am pretty sure you must use third-party software on ANY device.
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You have the right to make that evaluation. But I will merely note that while it may not match your needs, there are a great many people who don't seem to find that a great burden.
People act like EPUB is some sort of Holy Grail...