Interesting. In your answer, you focused on the stream of customers walking in the door, rather than on the welfare of the burger flipper or barista. Do I expect a burger flipper working at a hot grill to be just as safe in hour ten as hour eight or hour six of a given workday? No, I expect that burns, for example, are more likely at the end of long shifts than the beginning.
In addition, since humans are not machines, I expect that an extra day off might make workers more prompt and less likely to take six days at the beginning or end of a workweek.
At the time the trade unionists were fighting for our rights, the question wasn't burger-flipping, but generally factory work. The speed and accuracy at which a person working in a factory could produce goods safely diminishes beyond a certain point, and while it's probably closer to five or six hours for peak efficiency, employers were demanding ten- or twelve-hour days at the time, so a 50% reduction was a bridge too far. Still, through a combination of many different kinds of protest and some documentation that safety incidents climbed dramatically as workers grew more tired, they managed to get hours pushed down to no more than eight as "normal," with anything beyond that requiring overtime pay.
Henry Ford took the reasoning of the trade unionists, who had successfully pushed him into eight-hour days already, and took it farther. He reasoned that giving workers two days off instead of just one would enable him to push them harder, and get just as much work done while making happier employees, more loyal to him. Which worked out well for him, and 13 years later became the norm by law.
If you're talking about work which requires workers to be physically present for a set schedule, then yes, clearly capping each worker at 80% of hours would mean needing to increase staff by 20% to achieve the same result. But most businesses in which physical presence for given hours is important already have flexible scheduling to handle days of the week and times of the day at which business is busier.
If you're instead talking about, as you say, "any reasonable white collar job," then rather than letting only bold or clever people to get their work done in 32 hours while attending meetings to avoid raising eyebrows, it seems more reasonable to make that the norm, so that everybody can do the same. Then the receptionist can also work four days a week, with someone else filling in, rather than only those less visible.