If you care about accessibility and not being ageist, they are terrible for people with disabilities or the elderly. You will almost see no old or disabled person using a self checkout line.
As a show of more anecdotal evidence, a recent large grocery store chain in my large populated city of 2+ million people experiment with going self-checkout only failed so bad (lost so many customers and people were complaining), they hired cashiers again to basically scan people's groceries for them at the self-checkout line. Now they are stuck with the worst of both worlds.
You dutifully wander through the aisles gathering your goods, You slide your card, operate the pin pad, carry your own goods to your car. These are all goal directed activities you completed for YOURSELF in order that you could consume or use the products you have so acquired. There is no fundamental difference between scanning your own goods and sliding a card.
You aren't paying for someone to scan eggs and put it in a little bag you are paying for someone to manage every step between where the hen laid the egg and making it conveniently available on a shelf 1/2 a mile from where you live.
You aren't getting paid for doing it yourself like you aren't getting paid for carrying your own goods to your car instead you are benefiting from a price point enabled by the degree of automation and self service that the store engages in. Its ironic that people simultaneously flock to stores that have even slightly lower prices while complaining about lack of help. Simultaneously driving and bemoaning the same trend.
Your anecdote about bringing back the cashiers for the worst of both worlds sounds like a buggy whip manufacturer gleefully cackling at the unreliability of early cars. I believe we both know how THAT turned out. Given that Walmart was doing inventory on all its socks by walking past the socks with a wireless reader 10 years ago I'm pretty sure even your can of baked beans will have a chip in the label before long and your self checkout experience will be literally consist of solely being asked to pay for the goods in your cart. At this point paying an entire body to baby site each transaction would be wasteful and silly as 99% of them will consist of you touching a button on your phone or on store hardware to pay.
You say that self checkout is "ageist" in an era where even people turning 60 years old today probably saw a computer by the time they were 30. In 10 years this will be true of our 70 year olds. Are we just supposed to pretend that people who didn't have a phone shoved in their hand at 5 can't learn? That would seem in itself to be ageist. The reality is that old machines sucked pretty badly and older people don't like change and have taken their impression from older machines. This isn't the same thing as being incapable. For those that truly do have difficulties it ought to be sufficient to have staff on hand to help.
There is fundamentally no difference between waiting in line 2 minutes patiently in line and standing at a self checkout while the attendant helps others there for the same duration but people don't seem to react the same at all. In fact properly regarded what having 4 self checkouts with one attendant instead of 1 cashier with one computer is the probability of waiting far less.
Would you rather wait behind 3 other people or would you rather checkout out immediately and wait 30 seconds if you need help with the machine?
Get better self-checkouts. The ones that try to simulate how a traditional checkout works are stupid, the same way a mechanical messenger pigeon would be stupid but email is pretty good.
A friend of mine has several serious problems. He much prefers the self-checkout. No human interactions, which means no need to try to figure out what the other person thought they were communicating.
It was much better during the worst of lockdown too. I go in, I pick up items I want, I scan them and place them in my backpack, I go to the checkout, I hold the same device I used to scan items near the checkout, it acknowledges that I agreed to pay for the items I scanned, I put the backpack onto my back and I walk out of the store. Minimal contact, no human interaction, very low risk. Nice.
Or trying to do multiple scans and then bagging them all in one swoop.
And god help me if I try to scale with 2 more hands.