It is interesting how people serious about text editing always seem to end up throwing away most of the DOM and use Canvas or SVG (e.g. CodeMirror). But even then the browser brings a lot to the table in terms of a common runtime.
It brings a runtime that's easy to distribute to, that's pretty much it. I think there are plenty of runtimes out there (e.g. Java Web Start + JavaFX) that are just as good if not better, but they don't have anything like the same installed base (especially in locked-down corporate environments where you can't just distribute your thing bundled as an executable, which is where a preinstalled runtime really matters) as the browser.
As a user, I'd prefer a minimalist office suite that still doesn't have the browser as a starting point.
I think a browser is actually not a good UI for an office programs, because it really awkwardly competes with your own document model: HTML at its core still wants to be a document, except probably not the document you want to have. So you have to reimplement document APIs on top of already existing document APIs.
E.g., tracking and working with selected text is extremely important in office programs, but a browser already has built-in management of text selections: Except, the built-in management doesn't know which part is your chrome and which is your document, making you to either implement your own selection mechanism in addition to the browser's or handle all kinds of really weird special cases.
And so on...