I don't have any objections to the OS having ebook reader out of the box - I just think that it would be better as a separate app, rather than a browser. Browser is for, well, browsing the web. Reader is for working with ePub files, for starters - even in the scenario which you describe. And it can still be a basic viewer app that just opens on double click, with advanced functionality (like a library) available for those who need it.
To me, it's the decision to shove that functionality into Edge that feels like a typical developer shortcut: they wanted ePub, and ePub is basically HTML, and Edge already does HTML, so let's do a reader as an extension! But it doesn't really make any sense to me UX-wise. Again, the only reason for something to be in the browser like that is if you routinely open documents of that type via links. And I just don't see people doing that with ePub - note how your scenario doesn't involve anything of a kind.
To me, it's the decision to shove that functionality into Edge that feels like a typical developer shortcut: they wanted ePub, and ePub is basically HTML, and Edge already does HTML, so let's do a reader as an extension! But it doesn't really make any sense to me UX-wise. Again, the only reason for something to be in the browser like that is if you routinely open documents of that type via links. And I just don't see people doing that with ePub - note how your scenario doesn't involve anything of a kind.