Are we to believe that a 100 year old bizops practice is the optimal solution?
I honestly think that the main reason it's (nearly) impossible to implement something new, is because the old practice and mentality is so ingrained and etched in management, they'll feel cheated if the need to pay the same for less days (even though hours may be the same).
They'd rather have people around for as long as possible, doing 70%, than having them less but delivering 90%. The same type of management that values facetime and butts in seats over nearly anything else.