Interestingly, population growth actually does slow and disappear as countries become more 'advanced' economically.
Japan for example has a birthrate below 2, left to their own devices they might actually just die off eventually.
Europe and the US are both around the rate of replacement for birthrates. If the trends were to continue, they would go extinct.
3rd world countries are far less comfortable, and don't seem to be in any danger of extinction though lack of reproduction.
You seem to miss the idea of individual variation - motivated individuals existing in group X does not contradict the claim that members of group X (2020 USA for example) are less likely to be motivated than members of group Y (1800 USA for example).