Complexity, in Ousterhout’s sense, is primarily a term related to the difficult of making changes to a system (Complexity is the greatest limitation in software development). As such, the impact of highly complex modules that rarely need to be changed is very low. A rough equation is presented, where is the complexity of the overall system, is the complexity of any given part, and is the time spent making modifications to that part:
This loose formula carries the corollary that isolating complexity is much the same as eliminating complexity.
References
Ousterhout, John (2018). A Philosophy of Software Design.