> Why pay 3/4 of the population to work 40 hours a week when we could pay all of them to work 30 (then 20, then 10)?

Because corporations want to pay 60% of the population to work 50 hours a week on salary. This is why everyone should be hourly. That's the first step. Then, yeah sure, hourly with overtime pay for anyone working more than 30 hours a week.