It takes significantly longer to train an individual to be effective in any single job than it did in 1870. Even with a generic undergrad degree, pivoting from one job category to the next takes time. That time to pivot will only get longer as jobs become more specialized. When job categories are eliminated en-mass, you'll have large sways of people who aren't effectively employable any more, as the training required to pivot will just be too time consuming to merit the societal or personal cost to incur. Instead, they'll be net deficits, either through death (no safety blanket), feed off their security blanket if they have any, or feed off the system.
Of those 50% of farmers in 1870, how many ended up changing careers and finding better lives outside their work upbringing? Did everyone start to work in factories or was it the more likely outcome: Farmers stayed farmers till they retired/died, and their children were educated and left their agricultural roots to find different more intellectually challenging pursuits in cities. The farmer's safety blanket was that they were largely self-sufficient. Unless they had a big choaking loan from the bank, they could often coast into old-age off the value of their farms. People fired/downsized from Walmart, Disney, or TaxiCo's may have 401k's, but others will have literally nothing to fall back on. Yes, it is certainly a matter of poor judgement, but a popular failing. Now that automation is essentially eliminating most manufacturing, large sways of grunt labour and (with self-driving vehicles) huge chunks of our logistics needs, where will 5-6 billion people find to do with their lives which are 'valued' in a GDP sense?
I'd love to know, so that I can jump into that field now. Because even in software development, one of the apex demand fields, I see oversupply (of not so amazing candidates) and increasing automation to swallow their future prospects. If you can 8-ball be the winning growth job categories for the non-super-rich even 2 decades ahead, I'd believe that someone has answers worth sharing. The only answer I have is health care, which will hold until most of the baby boomers are dead.