> Traditionally, UNIX commands were written under the assumption they were going to be used primarily by other programs. They had more in common with functions in a programming language than with graphical applications.

Not quite. They were primarily intended for interactive use within a login shell. There are the programs which generate output on stdout (ls, cat, find, tty, who, date), and there are the "silent" text filters (tr, grep, cut, uniq, sort, wc). A one-liner would enable you to do basic computing tasks in that era. Any complex program would be written in C. After the appearance of DSLs like sed and AWK, certain string-heavy programs were offloaded to the shell.

The shell is not a sane programming environment and was never intended as such.