This is from 2013, and the original title is "Functional Programming in C++". Moreover, Carmack never makes a statement that functional programming is the futur

This is from 2013, and the original title is "Functional Programming in C++". Moreover, Carmack never makes a statement that functional programming is the future anywhere in this article. The closest he says is "Formal systems and automated reasoning about software will be increasingly important in the future," which is a markedly different sentiment. That said, the problem of not understanding possible code states does not seem to be purely a result of the internal software structure, but how it interacts with outside components, including the OS (if any). There is a combinatorial explosion of the state space as a program's dependencies increase. As such, the most practical way to deal with this is crash-only software whereby components do not perform complicated error recovery, but are simply fenced and restarted to known good states with persistence used to ensure that the component can return to its prior point of operation quickly. FP is useful for writing software like this, but not necessarily pure FP or even a single paradigm.