There are some really powerful UIs that are extremely flexible, like spreadsheets, or ComfyUI, or certain issue trackers. On the whole however I would say they

There are some really powerful UIs that are extremely flexible, like spreadsheets, or ComfyUI, or certain issue trackers. On the whole however I would say they all still are mostly their own interaction silos and integrations with other programs have to be explicitly built for each app. The great thing about a shell is that you can glue together programs that don't know about each other. This is not a silver bullet, with shell programs being text oriented etc., but it does give rise to some powerful possibilities that don't really work with UIs.