Added to that, DDT was being used in very high amounts as a pesticide, but didn't need to be used in nearly as high levels for suppressing malaria.

Resistance to DDT has formed anyway: http://en.wikipedia.org/wiki/DDT#Mosquito_resistance – though again this is because of agricultural use, not from disease prevention use.

On the other hand, it's not that clear to me that DDT has actually been discarded?