Maybe we have different understandings of REPLs. For me, a REPL is a tool that I can use to try out some portion of my code, see how it responds to various inputs, see how it handles certain cases, and explore the internals of it via debugging tools.

But... that's what I do in a unit test anyway. A unit test is essentially a REPL session that I've frozen in time, can replay whenever I want, can debug, can trigger a fresh run whenever changes occur (which isn't quite hot reloading, but often a darn sight more useful), and can keep track of and share with my team. Which means I'm not just able to explore things on my own, but I can see the explorations that other people have made with their code.

For me the REPL is the experience similar to Lisp Machines, Smalltalk kind of experience, which is loosely captured in Jupiter Notebooks.

As for unit tests being more usefull, depends on how much one is willing to wait for them to run in Rust, given its compile times.