Something to keep in mind is that calibre's ebook-viewer is in the middle of drastic changes.
It is being ported to use a single codebase across the desktop viewer and the calibre-server browser based reader.
It is being rewritten from scratch using QtWebEngine, instead of the current QtWebkit viewer.
It is gaining new features like annotations.
The ebook-viewer is generally targeted at being a user-friendly, featureful desktop ereader software, and I guess it should do a fairly good job of showing how a modern, compliant ereader functions.
(I mean... we already acknowledge that just as calibre does not show how some ancient boutique ereader software will render your book, Sigil is not going to either, because implementing device quirks is a fruitless, thankless waste of time.)
Does Sigil need to develop its own competitive ereader as well? Maybe it would make sense to recommend calibre for this (and make sure that it is suitable for such use). calibre has always been more of a go-to for reading and organizing ebooks, while Sigil has always been more of a go-to for editing them. Just because calibre grew an editor doesn't mean Sigil *needs* to grow an ereader.
If Sigil did get an ereader, what additional use would one get out of it that calbre's ebook-viewer is not likely to handle? Maybe Kovid could provide some insights into what the upcoming calibre 4.x webengine-based viewer can do for us?