IMHO it's also an useful illustration that being able to replace people is asymmetric.
It may be that Bob can do Joe's specialization at half speed, but it's plausible that Joe can't do Bob's specialization properly at all, even after a year of training; so replaceability only works one way.
And I have worked with engineers that could do improvements to an unfamiliar component/subsystem faster than the current engineer who is the most familiar with it; i.e. their time of learning+adaptation+implementation was faster than the other persons' implementation alone. And it's not that the other engineer was incompetent, they were quite reasonable.
It may be that Bob can do Joe's specialization at half speed, but it's plausible that Joe can't do Bob's specialization properly at all, even after a year of training; so replaceability only works one way.
And I have worked with engineers that could do improvements to an unfamiliar component/subsystem faster than the current engineer who is the most familiar with it; i.e. their time of learning+adaptation+implementation was faster than the other persons' implementation alone. And it's not that the other engineer was incompetent, they were quite reasonable.