[mrloba]

This is is the crux. It's possible to justify any and all designs using the single responsibility principle / separation of concerns. I've seen plenty of bad designs rationalized this way.

Answering your question (or "what is _one_ thing?") is often called an art, and seems to be learned through experience. Though I do wonder if it's possible to teach as well. So far in this industry we teach it using rules of thumb and "principles", but it doesn't seem to work very well.