What do they do about people who are already making $15? Will they get a raise, or will they suddenly have a minimum wage job?
They won’t get raises - and many people who currently earn less are going to find themselves out of jobs as companies wring more work out of their better employees to justify paying them $15 per hour. This probably leads to a very real case of “women and minorities hit hardest”, as companies that shrink their staffs to meet the higher hourly wage requirements are basically going to keep literate people with strong work ethics who won’t take maternity leaves.