Certainly in the more tourist-y coastal resorts throughout the med, including Spain, Portugal, Cyprus, there is usually at least one large supermarket open on Sunday, usually one of the more modern/larger "out of town" supermarkets and they're usually busy. At the other end of the scale, the smaller/convenience stores on virtually every street corner especially near beaches/hotels are also often open 7 days. It tends to be the "mid-sized" stores that close. We always go self-catering in a villa/complex with extended family and have never yet found nowhere to shop on a Sunday that's not within a 5/10 minute car drive.
I think it's good for the shop's trade as they've a "USP" - i.e. when we used to go to Paphos years ago, we always went to the same supermarket in the centre - in those days (10-20 years ago), we didn't expect Sunday opening. We've started going back the last few years and googled for one that opens Sunday - found one that was about a mile inland, so we tried it out - far bigger, far better range of goods, easier parking, so that's what we use all the time now, and havn't been to the central one since, regardless of what day it is. If the central one had opened on a Sunday, we'd be still using it and wouldn't have even known about the inland one. Same happened last year when we went to Sitges near Barcelona - used a central supermarket for the first few days, then noticed it was shut on Sunday, so googled for the Sunday opening one, which we again found was only half a mile further inland, but again, easier/free parking, far superior product range and cheaper. So it's not just the Sunday trade they benefit from, they get extra trade on other days once customers have experienced them.
I think 7 day trading is fast becoming the normal and Sunday closures will continue to get rarer and ultimately a thing of the past.
