> Old doesn't imply arcane.
Indeed so. For example, awk is about the same age but doesn't suffer the same problems.
> the shell syntax was never meant to be anything like a systems or application programming language
A programmable shell is not only like a programming language, it is a programming language. Systems tasks have some specialised requirements but, overall, scripting is programming. There is nothing in the problem domain that requires the posix string-substitution problems.
If fish or oil had existed back then, they would likely have won, since they are better.
This is a logical statement that is technically true while not matching real-world experience. Scripting as a type of programming is different from application development programming, and scripting needs differences in features and interface to be convenient and comfortable.
> There is nothing in the problem domain that requires the posix string-substitution problems.
What do you mean? Environment variables and command line execution both separately lead to string substitutions. The fish shell uses string substitutions.
You emphasized ‘requires’ which again might make your statement technically true while still missing the experiential forest for the logical trees. You can use x86 assembly for your shell if you want, there’s nothing that “requires” posix. The reason string substitutions exist is because they’re useful and convenient; the alternatives are verbose and klunky.