MIT Technology Review writes: People are not the problem they are the solution. In 1900 there were fewer people in the world and they had a lower per-person standard of living than the average person today. Similarly the people of 1800 were not in general as well off as those of 1900, even though yet again there were fewer of them. So if you connect the dots and extrapolate a trend you would see that historically human standard of living goes up as population goes up. People are of value to each other, the more people the better off we all are.
Malthus and Ehrlich made predictions based on their theory. Those predictions turned not only to be wrong but 180 degrees off.
Basically the reason they were wrong is that humans are intellegent and the more people you have the more intelligence you have to apply towards solving problems.