Sir, your wisdom appears to contradict wikipedia, that specifically says this about Bourne shell (precursor to bash):
Although it is used as an interactive command interpreter, it was also intended as a scripting language and contains most of the features that are commonly considered to produce structured programs.
A reference is made to an
article, and from this I again quote:
The primary problem was to design the shell be a fully programmable scripting language that could also serve as the interface to users typing commands interactively at a terminal.
Now the history and evolution of shells is quite an interesting topic. Please do not hesitate in providing references, should you be aware of any well-researched works on this subject matter.
ps. Your exposition has vaguely reminded me of a magnificent oeuvre in quite a different field. Thoroughly recommendable, this. 