1) Trading hours restrictions cause needless traffic congestion at enforced closing times.
2) The vast majority of people attend no regular religious Sunday service, and no one has any right to impose their viewpoint upon others who do not share it. Not one extra person ever attended church because the shops were closed.
3) Extra employment and economic activity would be created.
4) Town centre streets would be safer since shops would be open and citizens other than drinkers in the area for longer.
5) Any expenditure required for enforcement of opening hours would be saved.
6) In highly populated areas, citizens are to be seen every Sunday waiting outside shops for them to open, as Cold War communists used to queue for bread. Now, as then, heavy-handed and arbitrary regulations get in the way.
7) Elderly people often meet casually in malls and supermarket cafes to chat and enforced closing increases their isolation – perversely in the guise of encouraging greater community.
Finally, this is supposed to be a free country. If there are better things to do on a Sunday, then please give the people the opportunity to decide what those things are. People vote with their feet, so if it isn't necessary to open the shops, their owners will close them for the sake of economy.
The past is a foreign country – this is the 21st century and we do things differently here.