Complexity is in conflict with usable composition. Because of simple, small interfaces, the surface area needed for composition to cover all cases in a plan 9 environment is much smaller.

This allows for things like remote login to be implemented as a small number of 'mount' calls, git to serve repos in a way that can be scripted without writing gobs of porcelain, or sshnet to trivially replace the entire network stack in userspace (namespace by namespace), so that software doesn't need to implement features like socks proxy support.

The simplicity and uniformity of the interfaces and tools is an enabler.

But, you're right. I should have picked scheme or lua as an analogy. Scheme is a particularly good one: the simplicity of its syntax enables easier macro manipulation. How many special cases would you need to have if you were implementing a lisp style macro system for C++?

Smaller interfaces and implementations, lead to smaller sets of special cases and contexts to keep in mind, easier interposition and emulation, and faster debugging.

> You present it as mutually exclusive.

I honestly have no idea what you mean. What is the "it" that I am presenting, and what do you think I presented it as excluding?