This guy sounds like a Luddite. It very well may be that cashiers will be a thing of the past, just as other jobs have been phased out such as elevator operators and gas station attendees. You cannot stop human innovation and progress. One hundred years from now there will likely be no cashiers in most large stores (if there are stores) and many other low-skill jobs will be replaced.
I have been a cashier and I have known some great cashiers who are great at their job. Since they take pride in their work, they will find another job and will be good at that as well because being a cashier is not their identity, it is their job.