I think it depends on the surroundings and the level of trust in the customers. Anecdote: when I was a child we had a small groceries shop in our street where I would go and get my sweets of stuff that my mum needed for cooking. So when the owner of the shop was not inside the shop I would just leave the money on the counter. Can you imagine this today in a big city at the main station?
I dunno, even the Migros at Zürich HB doesn't have that many people minding the self-checkouts that I remember.
But yeah, there will probably be shops where self-checkout will never be feasible, like electronics stores or pharmacies.
Pretty sure that's not generally the case. In Migros (Swiss supermarket chain), where I shop, they have one or two people monitoring 10-15 self-checkout stations. Even though the customers might be a bit slower scanning than full time cashiers, that's still a lot less people than an equivalent amount of old-school checkout lanes.