Take construction; you have labor, semi skilled labor, skilled labor, foremen, superintendents, project managers. Not every 'electrician' needs to be able to wire a box, or diagnose a problem; most need to run wire, set boxes, install switches/outlets; you really only need one guy per crew who is what I would called truly skilled labor. Most stuff is like that.
A grade crew, gonna have a shovel guy or two[unskilled] (who can hop on a roller, clean curbs, pull a string line, spot utilities, back up dump trucks), a loader man [semi skilled] (push rock/dirt, load trucks, pick up windrows with out gouging the base), and a dozer or grader man [normally riding the fence between skilled and foreman/superintendent]; the shovel guys are 100% replaceable (although, hard to find nowadays); the loader man, he can be replaced or a shovel guy moved up, but you'll miss the operator; the grader guy, worth his wait in gold; he gets a DUI, you give the loader man a truck and tell him he has to pick up graderman, he gets locked up for beating with girlfriend, you bail him out; although replaceable, the job comes to a screaming halt until he is replaced.
The admin world is the same way; everyone can be replaced but some, it's difficult.
The advantage of mass manufacturing, they can have a simple repetitive, training program, where guys can simply plugged in, trained, and the whole process moves on. Now, as you move up from the base floor line, your guys keeping parts, tools, ect coming in on time/repaired/replaced, much harder to plug and play.
