> If the productivity benefit from AI is real, there's no reason you shouldn't be paid the same for less time because your output is the same/better.

That reasoning makes sense in a world where people are paid in proportion to the value they create. But it doesn't make sense in a world where supply and demand determine the price paid for something. Because if you can do the same work in less time in means you've actually increased the supply of producing things and thus decreased its worth.