I don’t agree with this assumption that shell should use mechanisms we accept in “regular” programming languages, whatever that means. And why not the other way around? Why do we accept mechanisms in programming that we wouldn’t accept in a shell? There are layers of assumption in your assumption.
I also disagree with lumping scripting and programming together. Those are two different activities, and shell is better at scripting than programming, and it’s better at scripting than programming languages.
Try using python as your shell, and you’ll find out that python is fundamentally bad as a shell REPL. The shell is necessarily different from python or C++ or <your_favorite_programming_language>, starting with the ability to run command lines by typing them, with zero extra syntax. That is something regular programming languages can’t do, and it leads directly to the need for string substitution rules.
If you want a shell made out of a good programming language… make one! People have been thinking about this for decades and nobody has come up with a good one, because shell languages like bash are better at shelling than programming languages.