I think it is somewhat outdated, given that we now live in a society where people work long hours, at weekends and overnight.
Most shops are open on Sunday, so people who work in such retail environments are likely to be involved already. Its also clear that people like to go shopping on Sunday, especially when for many the weekend is likely the only chance they get, so its nice that it can be spread across the two days, rather than it being super busy on Saturday.
I think the people most likely to lose out are the small traders who are exempt from the laws. I've often had to go to a small local trader or a Tesco Express when main shops are closed... but saying that, giving money to Tesco is hardly a small trader.
Its interesting that the law will be suspended for 8 weeks during the Olympics, so that also goes to show that it seems out of date.