Scan & Go actually works really well.
You just have to remove the side cases where it doesn't - like loose products as you say. But loose products are, by far, not essential to anyone's shopping. I don't buy cornflakes by the flake or by the gram, there's no need to buy pears individually by weight either.
Since those scan & go technologies come in, however, my shopping is now utterly predictable, far more accurate, less impulse buys, and I'm in and out of the store in half the time. Mainly, it has to be said, because I don't have to unpack my goods onto a conveyor, have a spotty teenager scan them by hand, then repack them back into the exact same place I just unpacked them from. That, alone, makes it worth it for me.
Being able to see the exact price while I'm deciding on the product is another bonus. Whether it's labelled or not, I can see the price and know immediately whether I've picked up something that's part of a deal or not... which I imagine quite a bit of profit was made by companies before by "mistakes" on the human's part. I first started using it when I had no money, and it was great to ensure I was inside a strict budget. I could scroll back and remove that product from 4 aisles ago that would give me just enough money back to stay under my limit, as well as having a running total in front of my face at all times. I'm guessing that doesn't help profits either!
And I simply don't bother with loose goods. I can be in and out of the store in under an hour with an entire month's shopping in one hit, for cheaper than I ever could. I don't even bother to become a member of their special loyalty programmes, because increasingly they were becoming paid-for and because of my single-trip mentality, they actually weren't worth it after a while. I was paying, say, GBP9 a month, in order to get 10% off two shops. So if I spend GBP90 a month, I was profiting. But the programmes got more expensive, applied to less products, and increasingly the terminals wouldn't easily read my codes, discounts, vouchers, etc. and it became a nuisance. So I just cancelled it and actually saved money by doing so because occasionally I go to a different shop instead - their loyalty programme basically forced me to become "disloyal".
Manual checkouts ALWAYS queue. Self-checkouts there's a small queue but it moves fast as there are always more self-checkouts than manual staff. Scan & go never has any queue at all for me. Repeatedly, month after month, I just literally scan-and-go. Along with the packing/unpacking it cuts my shopping time in half, and even the time lost to typing in barcodes, etc. for vouchers and discounts is such a factor still!
Self-checkout is a nonsense outside of tiny few-item purchases - it's for people buying lunch in a store while they are out of the office, that's what it's for, and it works fine for that.
Scan & go is great, but it's for buying a certain subset of things (i.e. no loose items, nothing that requires verification, etc.) and keeping an eye on the bulk.
But manual-checkout... I can't find a use pattern for any more. It's just slower, more expensive and less useful every time I'm forced to use it. I'd rather avoid it and do those bits some other way and somewhere else. If my local supermarket was just scan-and-go, or like these Amazon stores, I'd actually be happier. Even delivery isn't as efficient because they insist on making constant substitutions (what happened to accurate stock control?), and then I have spend ages making the shopping list, be in at a given time (or "click and collect", which is actually less efficient than just scan-and-go), pay a fee for delivery, deal with trying to not delay the driver by getting everything into the house quickly, etc.
Of all the options I want in my future when I get old and infirm and want to just take my time and buy stuff I want without extra nonsense (or unpacking repeatedly), scan&go and home delivery are the options I want, and home delivery will still cost money.