The "vt100 teletype emulation" only concerns the protocol in which the program describes what to draw on the display. It might be inconvenient for the programmer, but for the user it is kinda irrelevant.
> Such statements can only be voiced by those that never experienced what using a proper REPL actually feels like.
Maybe. What do you mean with proper REPL? SBCL just runs fine in my terminal emulator. As do all other kind of programming languages, like Prolog.
When I interface with the OS, I want to start programs, control the processes, setup communication channels (pipes) and tell to computer to combine multiple programs to filter, combine, split stuff and redirect it to files and other programs. These should be also be able to run several times with slight differences. All of that works just fine in my shell. What are you missing?
SBCL REPL cannot do this on xterm, it needs a proper hosting REPL environment like SLIME, which is no wonder, given how Emacs came to be and the interaction with genera.
You are missing integration with live debugging, calling anything on the OS during a REPL session, e.g. OS APIs, calling into automation points of the OS,
Can try out directly on the browser, courtesy of WebAssembly and recovery of Xerox PARC software, https://interlisp.org.
Or get either Squeak or Pharo, and see how using the Transcript integrates with the whole platform
Windows, with either PowerShell ISE, or its new VSCode integration, are the closest to these kind of experiences.
To finalise with Xerox PARC view on UNIX, from 1989
"UNIX Needs A True Integrated Environment: CASE Closed"
Such statements can only be voiced by those that never experienced what using a proper REPL actually feels like.
Not something that emulates a vt100 teletype.