You're talking about cross-cutting UI concerns (themes, widgets, window system), which I agree are largely mature, while I'm talking about vertical combinations of specific UIs and subsystems, which aren't mature by a long shot.
Example: "delete file" on Windows. Windows has relatively aggressive file locking, but if you try to delete a file that's open, it just errors out, it doesn't tell you what's using the file and give you the option to (force) quit it.
See also: networking, disk management, permissions, sound, preferences, ipc, etc. There are a hundred of these "just needs a bit of work" UI verticals, but designers keep trying to solve these problems with themes and functionality subsets, which are doomed to fail because the underlying issue is a lack of expressiveness in the existing UI, not an excess of it. Meanwhile, systems people live with "just use CLI / sysinternals / wireshark / nmap" like it isn't an issue that bog-standard tasks still require arcane tools in 2020 (arcane by the standards of typical users).