I would imagine that it's reached the point of diminishing returns -- it would take months of hard work to squeeze another 1% compression out of the format, in

I would imagine that it's reached the point of diminishing returns -- it would take months of hard work to squeeze another 1% compression out of the format, in which time hard drive space per dollar has gone up by 10% with no effort (on the part of the software developer) at all. (There are probably still compression technologies waiting to be invented; but applying them within the constraints of an existing format is hard or impossible, and creating a new format means you need a /massive/ improvement to make switching attractive for the consumer)