I feel like software dev has a very steep slope of diminishing returns on adding new people, mainly because of how complex the projects are. It's very different from adding a new person to an assembly line, because you need to know more than just how to do your one part.
Funnily enough though in software engineering we think of ourselves as practicing the art of transforming human creativity to machinery logic, we're really bad at finding good measurements for our work. Productivity measurement? Code quality measurement? Contribution measurement? We're bad at measuring them or downright don't know how to measure them!
I don't know how steep the diminishing returns are in software engineering when adding people, but I do agree with you that software engineering is much more than the sum of the efforts of X number of people
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