> A small but illustrative example: given a live running clisp program, I can click on a UI element to inspect both the live object and the underlying code in the IDE. I can even copy and paste UI elements!

I agree GUI is awkward in Emacs/SLIME. I think the reason is there isn't a standardized GUI framework across the Lisp world. Otherwise people can make SLIME support it.

> If I wanted to use slime-fancy to inspect a class, what I get is a dead list of raw text that I have to awkwardly interact with.

This part I do not agree. Nowadays "text" in Emacs/SLIME is far from "raw text", there's button that respond to hover, with right click context menu, and can be copy and pasted. I recall some small quirks (like objects in inspector are not presentations) but the only reason they aren't fixed (yet) is nobody get bothered enough. There's rarely any fundamental limitation. After all, it wouldn't be fare to call LispWorks UI "dead pixels".

BTW I once wrote yet another over ambitious project < https://github.com/neomacs-project/neomacs > but it doesn't go anywhere either. People do not care enough, Emacs is good enough.