Shell is a command-line interpreter with additional functionality like scripting. Why it should have fully functional REPL? Lisp/ST/etc are programming languages, they are uncomfortable for copying files, moving directories, calling curl or using tar.
> An approximation of it would be to have a shell that is a Jupyter netbook with access to the whole set of OS APIs, shared libraries and applications, with the ability to jump into the debugger at any given step of the pipeline, and redo the current step.
You can use python for this purpose. I don't understand why Unix shell should have all this functionality.
> I bet you saw GUI apps on UNIX workstations, though.
Yes, of course, I have GUI apps on my MacBook, but they don't use piping for inter-process communication because bi-directional pipes are not quite useful like an ordinary RPC.