On the same line, let me recommend you a book: "The Design of the Unix Operating System," by Maurice J. Bach. If I am not mistaken, Mr. Bach was a member at Bell Labs.
My personal take on the book is that the kernel was meant to be a portable virtual machine, extensible through processes, and that those would be the building blocks of user applications for which the shell would act as glue.
In other words, the shell and the OS are separate because most of what we commonly call "the OS" may be interpreted as mere encapsulation of less versatile hardware architectures.
