I thought the point of bendable panels was to move to thinner plastic substrates. Wouldn't this be sufficient to reduce the mass? Presumably they could also reduce the thickness and mass of the frame that supports the display if the display is more durable. Reducing the size and mass of the PCB and associated electronics could also result in a lighter device.

What's the point of this if a lighter device isn't a higher priority: To facilitate the use of larger batteries without altering the form or substantially altering the mass, of course.