But GUIs never took off as a UI for a general purpose computer, they became the UI for application on a general purpose computer. For them to be the former requires them to be programmable. Smalltalk is the best/most-famous example of a Graphical UI for a general purpose computer I can think of...

The main point is that for a general purpose computer the UI needs to integrate programming. Programming is how you use a computer. The shell (text) is currently the primary UI that inherently allows programming.

Is a modern phone a general purpose computer?

What kind-of UI does a modern phone present?