Will many of today's unemployed be permanent casualties of America's long-range industrial retrenchment? That is the awesome and unnerving question asked by another of the superb ''NBC Reports'' economic documentaries: Marvelous Machines . . . Expendable People (Thursday, 10-11 p.m., check local listings). The program's answer, unfortunately, seems to be a qualified ''Yes.''
Produced by Jim Cannon under the astute guidance of NBC's veteran news president, Reuven Frank, ''Marvelous Machines'' is reported by correspondent Edwin Newman - NBC's equivalent of Bill Moyers - who searches for trend and significance while never lacking in compassion.
''Even with recovery, as industries automate, it is expected that at least 300,000 factory workers will have been permanently displaced by 1985,'' intones Mr. Newman as he visits ghost steel towns, chats with unemployed - and perhaps unemployable - factory workers straining hard to learn computer techniques.
To compete in the international markets, it is clear that American industry must modernize. But, comments Newman, to modernize ''they must install machines that will inevitably displace human beings. And they must slim down, concentrate on what they do best.
That may be their salvation. But the program suggests it may also mean disaster for a large part of our blue-collar labor force - the working middle class. Will they become a lost middle class, depressed and resentful? Or will they share in the benefits of high technology?''
The documentary sets up its premise and then proceeds to delve deeply into all aspects of the problem by talking with academics, economists, corporate planners, and labor leaders as well as ordinary blue-collar workers caught in the transitional squeeze. The answers are not easy or obvious - the possibilities are varied. Just about everybody agrees that the problem exists and it cannot be resolved in the way that one worker plaintively suggests: ''All we want is it to be the way it was. That's all. Just be the same.''
''Marvelous Machines'' has a potentially optimistic alternative. ''It may all be part of an economic transformation that promises a bright new day. Old industries dying, so that new ones can rise in their place.'' The late economist Joseph Schumpeter once called this transformation ''creative destruction,'' explains Newman.
New industries like the computer industry are already taking their places in the economy. The documentary compares the computer to the steam engine, which ushered in the Industrial Revolution two centuries ago. But is this second ''industrial revolution'' too fast for our own good?
''It took four or five generations to move from agriculture to manufacturing, '' says economist Barry Bluestone. ''We're now seeing the transition between industries . . . occurring within a single generation, sometimes occurring within only ten years.''
Still another ominous note is sounded by Massachusetts Institute of Technology economist Harley Shaiken: ''Micro-electronics severs the link between a company doing well and the creation of jobs. So, coming out of this recession - and this may be our first high-tech recovery - we're seeing the situation where some companies could be doing well by not hiring workers.''
This thoughtful, incisive documentary comes to the conclusion that the fundamental structure of the economy that gave us Middle America is disappearing. In its place, according to Bluestone, we are getting an industrial distribution which has greater degrees of inequality and earnings. ''And we can expect a major change in the political, social and cultural structure of society.''
''Marvelous Machines'' points in many directions in its revealing search for acceptable solutions. One thing seems certain: American blue-collar economy will never again be ''the way that it was.''