I can't find it at the moment but I remember reading a study showing that even with the cost of fixing Y2K issues it was still an economically sound decision to use two digit years given the historical cost of storage. In other words it was cheaper to fix the problem in 1999 than it would have been to buy enough mainframe storage in 1979 to store the extra two digits.
Hah! Yeah, I could definitely see that being true. It was hasty of me to refer to that as "a mistake": as you say, that was a world in which every byte was several orders of magnitude more expensive.
To save a similar amount of money on storage today of course it'd probably have to be tens or hundreds of kilobytes wasted per record. That kind of an optimization would very often still be worth looking into and optimizing for.
