Screwball Ninja had some issues with this aspect as well that raises more questions about this whole concept.
But there are more fundamental issues: like the fact that Emma apparently needs to “believe” in fairy tales/the book specifically or everything collapses. Does she need to believe during THIS curse in particular? Because Emma was cursed to normality in NY for a year and all the other realms kept on keeping on. And why does burning the book destroy the other realms? There’s a whole bunch of blank books in the Sorcerer’s Mansion (see: S5 finale); I thought the book just recorded what had already happened. And even if burning a book destroyed entire lands, shouldn’t that be something that’s established EARLIER IN THE SEASON, AND NOT IN THE SECOND PART OF THE FINALE? We see the BF try and get Emma to burn the book for an entire episode before we learn what that means. How can we care if we don’t know what’s at stake?