> It's really not. A service adds complexity. If you have no reason to add it besides to "reduce complexity" - that is an oxymoron.

No, it really is. I can just as easily say that a system is simpler when it's composite parts are isolated, or that a system is simpler if I can take one component and reason about it in isolation, etc.

Similarly, I could say that libraries add or reduce complexity by making similar appeals like "a single unit of code is simpler" or "separate, smaller units are simpler" etc.