It is absolutely about scale.... an employee costs $x of dollars regardless of how many servers they are managing, and might improve performance y%..... that only becomes worth it if y% of your hardware costs is greater than the $x for the employee.
The issue is extra employees run out of low hanging fruit to optimize, so that y% isn’t a constant. Extra hardware benefits from all the existing optimized code written by your team, where extra manpower needs to improve the already optimized code already written by your team.
