if your cache line is 75% zeroes, then half of it respects the objective of "should contain 50% of each bit", and the other half is all zeroes, which is a waste

if your cache line is 75% zeroes, then half of it respects the objective of "should contain 50% of each bit", and the other half is all zeroes, which is a waste (according to this objective)... in other words, only half the cache is 'correctly' used. This is just another way of saying that your D$ size is effectively halved.