Looking at the bigger M365 vision of embedding snippets of documents/chats/stuff-from-the-graph into every other asset. Having there a free-variable like a third party browser will make this a horrible thing to manage. Same also goes for App Store deploy to desktop: a stable html/css/js SDK is needed there as well.
I would absolutely hate Microsoft's monopolistic behavior, but this thing, IMHO, it is not. There are better example (e.g. what they do with VS Code or the .NET Debugger/HotReload) then this concrete case.