If I remember correctly, it was McDonald’s who first created the system where a person was used for one very simple task, and they would repeat this all day. In this way, their staff became easily replaceable and can therefore be paid a pittance.
Said another way, McDonald's figured out how to create a very simple to learn and do job, and created tons of new jobs that people without skills were finally able to get.
Is there any evidence to suggest these people were unemployable before McDonald's came up with this plan, like you seem to be suggesting, or that they'd be incapable of learning several roles within the "restaurant"?
Just the fact that they are working at McDonald's doing the very basic simple stuff, instead of doing something more complex. Remember, there weren't very many menial labor jobs before. You could dig ditches, paint stuff... Used to be you could do those menial repetitive jobs at a factory, but robots replaced a lot of that. And I think even retail is more complex than making burgers.
And what evidence is there that this work-plan created more jobs than would have been available with people doing various roles?
Self evidence again, just the fact that so many of these fast food jobs exist that wouldn't have otherwise. But, I guess no concrete evidence. It's impossible to predict what kinds of jobs would have existed if fast food never came about.
Also is there something wrong with training people without skills and then giving them a decent wage? I'm sure most people could manage a few simple roles rather than just one (with a little more training, which has to be given in both scenarios anyway).
Of course not. McDonald's even has their own college, called Hamburger University, just outside of Chicago, and training is mandatory for owners, and available for managers. But most people just aren't interested in learning. Plus there's not much to learn with just making burgers. Those employees would be better off taking the free time of their part time employment learning something else. Also, if we train everyone do to more complex things, who will be left to make the burgers? (Probably the same people who don't care about training now, since those who want extra training are already seeking it out)
So, same amount of training and the same number of jobs. Only difference is that in one scenario the staff are highly expendable.
Said another way, in one scenario, the job is so simple anyone can do it, and dozens of people are on the sidelines hoping for that job.
The job wouldn't be expendable if there weren't many other people looking to get it too. That's what determines if the job is expendable or not: how complex it is and how many other people are available to do it. Want a job that's not expendable? Learn a skill that can't be replicated by every single person who walks off the street.