Im fine with a shop thats open late morning til mid afternoon on a Sunday. Really don't see the point in 12-6 at all. Useless for popping in when getting ready for an afternoon out etc.
I also don't see how Sydenham Sainsburys is seen as a simple alternative, its not "just down the road" at all. Certainly not for those without a car.
Besides, there are later opening alternatives like the Co-Op and Tesco. While I understand that its convenient, its just that... Not a public service who should chop and change their staffs hours all the time because people cant remember to get something from the shops.
First world problems eh! Once upon a time the shops did half day on Wednesdays, closed on Sundays, and Sainsburys only had one late night a week... The world managed, and people tended to remember what they needed from the shops