Well, most people are replaceable, just not always by machines.
There are many jobs (especially in the automotive industry) where a machine can be more cost-effective. These jobs have to have absolutely ZERO human factor involved.
By zero human factor, that eliminates all Quality Control, Team Leader, Supervisor, and many machinist jobs, believe it or not.
Jobs which require grinding or buffing will always be performed by a human because they require judgment.
Custom jobs will be next to impossible to run in many shops.
Basically, stricly dedicated shops (which there are plenty) would be the only ones that apply. And yes, humans are replaceable in those conditions. I'm not saying it's the right thing to do when you get to the personal side of business, but it may be the most cost effective financially.
WindDancer -- As far as unions go, I am somewhat familiar with the Unions in the New York City area, and they have a video out there speaking about NYC union work has been declining. NYC is a major player in our country's industry. Also, many states (like mine) are non-union.