You can have a nice career programming in FORTRAN -- until they want you to learn C. That's a new skill, and tough for some people to learn. But, if you bear down, you can learn C, and have another nice career in programming -- until they want you to learn Java. Now, we're talking Object-Oriented programming and a lot of people just never grasp it. But maybe you can learn it -- until the next language comes along.
Not to beat a dead horse, but you can be a good Sys Admin in UNIX, but if your company switches to Windows with Active Directory, you have a lot to learn. Do you go back to college? Nope. You learn it on the job.
If a machinist finds his skills becoming outmoded, does he have to change careers? Go to college? I don't think so. I think college is one way to give people job skills -- but vocational education, with on-the-job training is probably more effective for a whole lot of careers (both blue collar and white collar).