I mostly agree but the competition in the field will dictate that. Like you stated if it's features you are after and the sold party won't mind these small deta

I mostly agree but the competition in the field will dictate that. Like you stated if it's features you are after and the sold party won't mind these small details then maybe they (developers) are interchangeable. But e.g in science mediocre work will cost a lot if certain minor details are found wrong. That's whole reproducibility crisis in science is about. The reputation of a whole institute / department might be at risk if the mistakes are exposed.