Self service tills and scanners won't completely replace checkouts because not everyone can use one, and they still need staff to keep an eye on things, and help people who have a problem. They break down.
The hand held scanners have to be cleaned between each use.
But generally, some jobs will be replaced by machines. That really isnt the issue.
The issue is, what do we do with the people who no longer have full time jobs as a result of mechanization?
Its completely possible for society to change and just give everyone enough to live on and more leisure time, but attitudes will have to change for that to happen.
The problem is, the Citizens Wage smack of socialism and a lot of people don't like that.