> but somewhere around the turn of the millennium things... broke
The rise of electron as an 'acceptable' (to some people) substitute for platform-native applications and the myth of the possibility of the "cross platform UI" has accelerated this, really. It's quite unfortunate.
Seems like Electron would make a stronger foundation than native toolkits for Emacs-style flexibility, extensibility and composition. The DOM, for all its faults, charts a solid path between being flexible and still being structured, all while making it easy to change and extend from the outside. I've found it's much easier to write a browser extension that adds functionality to GMail—even without GMail actively supporting extensions—than it is to write a similar extension to, say, the native Outlook app (even with explicit plugin support from Microsoft).
I haven't paid much attention to the Electron world myself so I don't know to what extent anybody is taking advantage of this, but even if they're not, I expect it would be easier than with alternatives.
My first job out of college was developing software on the Macintosh in the late 80s/early 90s and I absolutely took for granted being able to count on the user interface to work in a sane and consistent way for 99% of all software. I fondly recall and very much miss that.
But there was a time when interfaces were unified, sometimes to an absurd level (e.g. applications having a File menu when they had nothing to do with files) but IMO that is still better than every app being its own thing.