but for a big business
I'm not worried about big business. They will survive. I'm worried about the small bakery around the corner, about the hair salons down the street, the flower shop in the mall and basically every small, independent shop that exists which is a crucial part of every community it's part of. And those small businesses make up a significant portion of the job market.
A 6 hour day at the same pay would absolutely annihilate any retail consumer B2C business that exists anywhere in the western world.