There are many jobs that are now obsolete, or will be due to technological progress. Not only are many 'labor' jobs being rendered obsolete, but so are many 'mundane' white collar jobs.
Toll-booth operators are obsolete (or will be)
Bookkeepers are obsolete (or will be)
Many back-office support-services can be automated or re-located
All cashiers could potentially be made obsolete
Many retail sales could be made obsolete (as people shop online)
Where do all these people go? During the last technological revolution, there was a relatively smooth transition from farming to manufacturing. But I honestly don't see how large swathes of the population that don't have specialized skills will fit in in the new economy.
Most of the people reading this comment will do just fine. But we face the really tough questions when dealing with the increasingly large number of people who don't have skills that are in the demand. And even if they all got the skills, employers simply wouldn't need all of them as labor becomes increasingly leveraged.
A couple professors fro MIT just wrote an e-book called 'Race Against the Machine' discussing this phenomena.
In fact, developed nations in Europe and the Americas are importing vast numbers of low wage workers to do low value per hour jobs where humans are still better than machines. It continues in the face of a prolonged severe recession.
So the emerging problem of no available work to do does not seem to be materializing. In fact, it's hard to imagine. Does every public park and private garden in your city look as clean, inviting, and pretty as it could possibly be? Is every street paved smooth without cracks and potholes? Does a bumper crop of fruit never go bad for lack of harvesters?
Is every home clean and freshly scrubbed all the time and is a freshly cooked meal always waiting? Does every corner have a crossing guard during school hours?
There's a lot of intellectually undemanding work to be done in a utopia if we want to pay for it.
Right now, we import foreign workers to do cheap labor but we could take unemployed citizens and subsidize their wages to have our own do those jobs instead. The unemployed citizens are living on private or public welfare and social insurance today but they could be working. We could kick in public money to make it possible to pay a legal wage with benefits for the jobs undocumented workers do and the employers would prefer to stay legal by hiring citizens. We could even document the undocumented and add enough subsidies to employ them too.
Economic change driving social change is disruptive. But let's remember that technological productivity is a positive change that makes us wealthier.
Unemployment that follows from energy shortages or global climate change, if those ever get serious, would be a much bigger worry.
That seems to be the trend, long-term. But you can't say 'most', not yet.
There are still many manufacturing jobs available in the US. My employer has an open reqs out for our six manufacturing sites in the States. We're not the only ones in our industry.
Why does this myth persist? It might be that these jobs are no longer in traditional manufacturing areas. A decaded ago we had two sites in traditional blue-collar areas: one in Mass, one in Washington. We closed them down and relocated the work to existing sites in Idaho and Wisconsin, expanded there.
Because the labor and other costs in the old sites were higher than the drive-past states.
So ... a guy in Mass sees a shuttered factory. A guy in Wisconsin sees new construction. Perhaps the guy in Mass needs to move to where the jobs are. We're hiring.
The fact that maid/cook wages have not dropped to $7.25/hour is strong evidence that unemployment is caused primarily by the unwillingness of many Americans to take available jobs. I'd happily pay $10/hour for a maid, but you simply can't find people this cheap where I live.
We don't need to subsidize people's wages to solve this problem. We just need to stop paying people not to work. After all, why clean people's houses for $7.25/hour when you can just collect unemployment for 99 weeks?
Note that where I used to live (Pune), plenty of people were employed as maids/cooks/etc without subsidies. They weren't paid not to work. Instead of welfare, they had the Gandhi Rural Employment Guarantee (styled after the New Deal).
Relative to what, exactly? That transition was only smooth in retrospective. It took many decades and (among other things) spawned a major political ideology and a superpower based on it dominating a century.