Should LLMs write in mainstream languages?

I guess I'm rather late for a discussion here, came across by accident while searching for an older post I remembered. Still, this is a question I've been pondering about lately, so let me post a few remarks.

Modern harnesses are one size fits all. I think I'm not alone in thinking that were I to desing a harness to fit my work habbits, it'll look very different from the mainstream ones. One could also argue, if you design a harness for an LLM, it's best to bootstrap early in the process, i.e. start using your own harness for designing itself, for then your work stays more grounded. So, you have a system that rewrites itself. Intuitively, Lisp and Erlang are very good at changing live systems. So, would you not want to do it in those languages? Nice to see others touching on this topic.

For me the counterargument is that Lisp and Erlang are untyped. If I were to work on my own harness, what I would like to try is more type-oriented development, where I sketch the types and maybe some function signatures and let an agent fill the rest.