You know... you can always undock the Preview Widget and resize it to cover Code View when you're in "reading mode." You'd still be able to use the ToC to quickly jump between sections and Preview will update (just like Book View always did).
Book View isn't the only way to see the rendered content in Sigil. You can even test that out without installing the newest version.
EDIT: a screenshot of what I'm talking about with Sigil v0.9.18(ish)
And F10 has always been a toggle for Preview, so the "immediacy" of being able to make edits on the fly wouldn't be affected at all.