Even aside from that, most employers for hourly jobs — restaurants, retail, factories, whatever — will still need to operate the same amount of hours, so they’ll either hire more people or have to pay overtime.
This wouldn’t cause a competitive disadvantage, because just like the original shortening from 12 to 8 hour workdays, everyone is in the same boat. Companies will adapt to higher costs and lower profit per employee, in the same way modern workers have adapted to automation+a decreasing real dollar pay per hour (i.e. inflation).
Not that all of this wouldn’t cause a shock to the system, because it would, but there is absolutely nothing magical about the 40hr/week threshold.
The 8 hour workday was itself a shortening from the 12 hour workday. I think we can all agree society as a whole, including hourly workers, is better off in every regard working less than 12hrs/day— even if technically an hourly employee would make more if they worked 12 hours.