It's a bit of a misnomer that every self checkout reduces one job.
I have a friend who works at a massive Dollarama who installed self checkouts a year or so ago. There was rumours of course someone would get laid off. Never happened - the employees that would have been on till just got reassigned stocking shelves and maintaining the store instead.
It's pretty rare that they are as nakedly craven as to lay folks off the day the machines get installed. Instead, they rehire fewer people as the inevitable turnover happens. It's also not as straightforward as one checkout machine = one job.
And people maintain those machines too. A (now ex, unfortunately) friend works for the company that fixes them when they bork up. He's a very busy guy and there's an army of them out there.
Absolutely they do, but the math is obvious that there's one mechanic for hundreds if not thousands of machines. It wouldn't be cheaper otherwise.
If you're upset about self checkouts, ask yourself if you use ATM's, pump your own gas, ever used an automated phone system, dialed your own phone number instead of asking an operator to connect you, got upset when a machine reset the pins at the bowling alley instead of a pinsetter employee.....etc etc etc ad infinitum.
Some are improvements on systems, others are simply companies doing away with the messy bit of having to employ humans. Dialling my own number is obviously a technical improvement that skips a step. ATM's are a bit complex, as they have allowed banks to justify ridiculously short opening hours, though online banking has generally taken over for 95% of most banking needs. Automated phone systems are infuriating messes where the end user experience is notably worse and it only benefits the cost savings for the company.
Automation and globalisation has unquestionably had significant negative impact on the quality of life for people at the bottom end of the middle class and down. It's been a trickle to date, but it's going to become a social crisis if we're not careful. At the moment, the two best ways for people without post-secondary education to earn a living wage are by driving trucks or working as a construction labourer. Those two industries employ millions of Canadians, almost all of whom have limited education.
Trucks becoming automated is now all but inevitable. It's only a matter of time, particularly with the rapid advancement of self-driving cars. Having trucks that can operate 24 hours a day will be too much of an advantage for any freight company to ignore.
Construction is a little further behind, but I can tell you as someone in the industry, there is a lot of effort being put into figuring out ways to minimise necessary labour. Whether it's making buildings increasingly modular, with site work being limited to assembly of pre-built components, or whether it's using human-operated robots, the labour demand will be lessened significantly.
What that looks like in a society nobody knows. It's easy to accuse people like myself who are worried about this of being luddites, and to point to other work that has replaced extinct jobs. But this is a new level of automation with no plan for those left behind. The only analogue I can think of from history is when Rome became flooded with slave labour in the late republic, throwing a lot of citizens out of work, including soldiers returning from the campaigns where those same slaves had been captured. This resulted in a huge amount of social unrest, civil wars, populist politicians, and ultimately the end of the republic. As conquest became less common, the economy righted itself somewhat, but it took hundreds of years.
If we're not careful, between automation and global outsourcing, we could have a huge chunk of our population equally disenfranchised and militant...