> The surface area of stuff that an OS has to do today is absolutely massive, and the dependency graphs are poorly understood even by the engineers who work on those systems. Innocuous pieces of functionality have a way of becoming load-bearing over time

It’s not that huge. I use OpenBSD and the code is surprisingly approachable and readable. And even with no support for runtime modules, it’s pretty easy to customize and strip out whole features. Current complexity in something like linux is stretching the subsystems to support a huge array of use cases.