The changes aren't being made by anyone whose set foot in the DKH Sainsbury's more than once a year, if ever at all.
Biggest overhead for the company will be paying staff. So it's easier to put the responsibility on the customer to scan and pay without input from an employee. Lots of people will gladly take on that responsibility (myself included). It's faster for me to scan my own items, pay, and leave without input from a member of staff. I also don't need to queue which saves me time. Is it a good thing? For me, yes. For others, maybe not. Some people would need assistance and probably enjoy the social interaction. Anyone who isn't tech savvy probably finds it frustrating as well. I'm not sure if those members of staff would have lost their jobs or been offered an alternative within the shop. Anecdotally, whenever I'm in Sainsburys it seems to be split pretty evenly between shoppers and staff picking up items for home delivery.
Changing the layout of the shop would also be a decision from corporate. In the past, I'd be in and out of Sainos in 25 minutes because I knew what I wanted and where it'd be. When they changed the layout, it was probably 35 minutes for 2-3 shops. Now I'm back down to 25 because I know the new layout. The point is, during that time when I'd be in the shop an additional 10 minutes, it was likely I'd end up in an aisle with products I don't normally browse, and it's likely I saw something new I hadn't seen before and I'd considered purchasing it, if not purchasing it. I bet Sainsbury's have statistics showing that they see an increase in sales in the month after a shop layout change.
As long as people keep shopping there, and as long as internal analytics deem these processes worthwhile, things will not change. Best way to make any sort impact would be to vote with your wallet, I'd assume.