I tend to agree with you here, the small business argument of being able to open Sunday's due to high cost of wages is a furphy....There are two parts to this.
For those who aren't trading on Sundays - Practically, these small businesses haven't been trading on Sundays because it's not profitable to trade on Sundays. They haven't even been able to break even. All things being equal 25% reduction on an expense item that has got less than 100% weightage isn't going to turn a business profitable enough on Sundays to make them open up on Sundays.
For those who are already trading on Sundays - mostly big businesses - cost savings won't necessarily have to translate to hiring more workers. They are already optimised. So, this is just a welcome cost savings, they would be happy to keep for themselves.