> If you structure your software simply and make it easy to reason about what is going on

Complex problems are not easily made simple or easily reasoned about. Something like systemd (or any non trivial piece of linux) could never be made simple or, with your hands down in the guts of it, easily understood.

Breaking systemd's functionality into a million separate pieces wouldn't make the complexity go away, it would necessarily increase it.

I don't know what the solution is, but "making things simpler and easier to understand" is a bit hollow. Its problem space was never simple or easy to understand (unless you think systemd just runs some things at startup).