That replaced mostly manual labor with mostly manual labor. The millions of people involved in the horse industry supply chain, went to cities and they and their descendants worked in factories. A horse required a lot of non-mass-produced infrastructure to support, the feed, the shoes, the buggy, the saddle. There were millions employed in this industry.
That was replaced by cars. And while industrial machines automated a lot of what used to be manual blacksmith/tradeskills work, cars were made from thousands of parts, which required a massive new supply chain.
Software automation is completely different, it scales and replicates in ways that don't require new replacement labor. WhatsApp serviced 1 billion users with 33 employees. That's scale.
So when self driving trucks kill 8 million trucking supply chain jobs and self driving cars do reduce the need for car ownership, for parking attendants, valets, meter maids, traffic cops, uber drivers, etc, low skilled people won't suddenly find those millions of jobs replaced with similar jobs with similar skill levels in another vertical. They'll just be....GONE.
And keep in mind, the "gig" economy largely replaced good, reliable, jobs with temporary, inconsistent, jobs. The industrial era, with it's big centralized unions, who regularized pay, the work week, benefits, pensions, et al, was not re-invented for the new service and gig industry.
The idea that technological advancement doesn't create structural unemployment is largely an article of faith. What it means to have a middle class, blue collar, skilled position with predictable work/life balance, income, and benefits has largely evaporated for a good chunk of the country. What comes next after automated logistics, automated factory assembly, automated deliver and self-driving vehicles? Hell, Google Duplex even shows you a lot of the frontdesk services can be reduced along with concierge services to only those instances that go off the rails.
We better start thinking about these problems and stop with dogmatic faith that something big will come along to employ all the displaced people. I'm talking about either universal basic income, government make-work, or something no one's thought of yet.