I think the idea was that the OS, programming language, etc should all meld into a cohesive UI that would range from mouse manipulations for the simplest messages to keyboard input for finer communication (and whatever other hardware we come up with later), and a gradual learning curve that everyone (including kids) could mature along.

Some aspects of this are elaborated in "Design principles behind Smalltalk": https://www.cs.virginia.edu/~evans/cs655/readings/smalltalk....