It would solve the long-form document problem. It wouldn't help with the editing however. The problem with HTML as it is today, is that it has long left it's document-markup roots and turned into an app development platform, making it not really suitable for plain old documents. You'd need to cut it down to a subset of features that are necessary for documents (e.g. no Javascript), similar to how PDF/A removes features from PDF to create a more reliable and future proof format.

They're basically zip files with a standardized metadata file to determine chapter order, index page, … and every chapter is a html file.

That's the idea, and while at it, we could also make .zip files a proper Web technology with browser support. At the moment ePub exists in this weird twilight where it is build out of mostly Web technology, yet isn't actually part of the Web. Everything being packed into .zip files also means that you can't link directly to the individual pages within an ePub, as HTTP doesn't know how to unpack them. It's all weird and messy and surprising that nobody has cleaned it all up and integrated it into the Web properly.

So far the original Microsoft Edge is the only browser I am aware of with native ePub support, but even that didn't survive when they switched to Chrome's Bink.