While I agree that it would be nice to solve the world's problems, what good would come from basically heaven on earth?
If humanity had nothing to strive for, what would we become? A world of couch potatoes who don't have to worry about obesity? A world of straight A Harvard college graduates who never had to study for a test? A world of rich people who no longer need to know the meaning of charity? A world of young immortals who no longer experience death or loss?
What would be our motive for advances in science, politics, or even advances in art and literature?
I too would turn down the 20 days of wishes followed by death... I'd much rather have 20 years of struggle in this glorious mess of humanity... doing as much as I can to change the world for the better. Maybe someone could choose their wishes carefully enough to eradicate hardship while keeping us humans human, but I don't think I would qualify.