In a normal functioning economy, discounts are given for bulk purchases. If I want you to come to my house to fix something in 10 minutes, the hourly is higher than for a job that takes 10 hours because those extra hours provide the service provider with more income, less overhead, more certainty, etc.
So people working under 40 hours per week are actually the flexible workers that are worth a HIGHER wage, especially since the company isn't providing benefits. So why not make the minimum wage $20/hour and if a company wants to contract a lower rate with you in order for you to be on-premises for a full 40+ hours a week, they can negotiate those terms with you.
I think that sounds fair.