Are you sure? Is it a measurable phenomenon, or our perception?
I don't know where you are on the globe, but here in the US, the trades have increasing shortages. Even employers offering on-the-job training are failing to find enough qualified* applicants.
The problem is that the social status of these fields has fallen dramatically. The guy who pumps septic tanks in my hometown bills more per hour than the lawyer, yet nobody there hopes their kid will skip college and work on septic systems instead, nor would any out-of-work paper pusher I know consider taking the job (even if it meant making more money than he used to).
Our middle class is most hurt not by the changing nature of middle-class jobs, but the changing perception of them. As recently as the 70s or 80s, the guy who fixed your septic, installed your air conditioner, or fixed your car was considered an important part of society -- the very image of middle-class America. Now he's treated like a loser for not having a degree.
How many people who identify with the American middle class go seek new careers that they and their peers see as lower class, even for middle class wages? I don't know, but my gut and anecdotal observation say "not enough". Changing careers is really hard for some people -- how many will make it while their friends and neighbors look down on them for working a dirty job?
* In this context, "qualified" is roughly equivalent to "will show up on time, won't steal or destroy stuff, will behave decently toward coworkers and customers, can read, can follow directions and learn to turn a wrench". So, not looking for rocket scientists, just basically hard-working folks.
OK, but the economy can only support a certain number of tin-knockers, plumbers, septic-tank cleaners, janitors, etc. These jobs exist because there are other people with other jobs that can pay for these services.
The reality is that when folks have bills to pay, considerations of "status" will only last for a few months. A few months of unemployment and depleted savings will make _anyone_ reconsider what jobs are "beneath" their abilities.
Finally, its not like anyone can be a septic tank cleaner. It takes a bit of knowledge, the ability to develop a customer base, significant equipment, and is restricted to certain geographic locations (where people don't have sewers).
