There are some major pieces missing from your analysis: legacy code and infrastructure, and lack of good documentation.
These can make for a massive hairball of complexity that can swell the number of people needed to support it.
This reminds me of a talk I once saw by a Netflix SRE, who showed a crazy convoluted mess of a diagram with thousands of crisscrossing lines going everywhere, and him screaming "No one understands Netflix!!!"
If you think Netflix is bad, you should see architecture diagrams for one of the major ad platforms; 30+ service sub-systems get encapsulated as a tiny block of the massive diagram. Anyone who thinks complex systems can be built and maintained by a skeleton crew hasn't worked on complex systems or hasn't been exposed to their full scope.
These can make for a massive hairball of complexity that can swell the number of people needed to support it.
This reminds me of a talk I once saw by a Netflix SRE, who showed a crazy convoluted mess of a diagram with thousands of crisscrossing lines going everywhere, and him screaming "No one understands Netflix!!!"