Well, for starters, reproduction =/= techniological development. If it did, the incestual and short-lived goblinoid tribes of nearly every D&D setting would be vastly superior to other cultures. No, it is more correct to say that the accumulation and application of knowledge leads to technological advancements. Lifespan does factor into this, but in almost the exact opposite way of that which you propose.
If anything, the fact that demi-humans live for centuries on end and in most D&D settings are already given to exploring superior technology with regard to forging weapons and metals, would lead me to believe that the typical D&D demi-human culture should be significantly more advanced than humanity. Elves, for example, can learn and implement something in one lifetime that will take humanity six or seven generations to learn.