I have worked in service/retail, and this argument doesn’t make a lot of sense. Most service/retail is actually 7-day weeks, but the workers average out to 5-day weeks with rotating shifts etc.
All that would have to happen is the workers now average out to 4-day weeks, with a similar level of pay (which is what the 4-day week advocates are asking for).
The 4-day week isn’t about office workers, it’s about everyone.