What makes this an appropriate metric? What about, for example, the ratio of total user interaction time?
The lower layers of the stack may not need as much programming attention compared to the top of the stack, but that's only because the stack is top-heavy in terms of programs produced.
But in a given stack, the set of applications at the top of the stack is still not particularly big (most users do not use most applications). On the other hand, everything that users do passes through the lower parts of the stack (most users use the kernel, drivers and the graphics layer constantly, even if indirectly).
