There used to be extra pay for working unsocial hours, including weekends, in the retail trade. Overtime also paid at a higher rate. Major firms brought in deals whereby staff could agree to sign those rights away in exchange for a lump sum of a few hundred quid, and then those hours were as far as possible only given to staff who agreed to that or new starters whose contracts paid no extra time rates. Which meant other staff lost out on the overtime, so joined too. So huge numbers of people now have worse pay and conditions than they did a decade ago, and it's just accepted and seen as How Things Are, along with fake "apprenticeships" (in the retail trade? Because working a till requires months of training at a lower rate of pay?) and 0 hour contracts. All extended hours would mean is bullying staff to never have time with their own families.
If we still had effective unions, then this sort of thing wouldn't be a problem. Staff could still be paid time and a half or double time for working unsocial hours, so plenty who are single or childless or with older kids would be quite happy to do it. But we don't, so it is. And in this context I'm glad they refused to extend those hours, because the truth is, staff would be given no choice at all in whether to work the hours, not in reality.
I also don't think the online argument holds up, because people choose to shop online for price reasons as much as anything else. It's not necessarily more convenient to get something online when it won't be with you for a week, but it's usually considerably cheaper. And Sunday trading won't change that.
The Entertainer don't open on Sundays because the owner is a Christian and says that staff should have their day of rest with their families. I admire that. And it doesn't seem to be trashing their business, either, does it?