"Just wondering. What would the world be like today if they had restricted development of the undeveloped areas of the earth, say 500 years ago?"

Well, first there wasn't much of *they* 500 years ago. And "undeveloped area" were extremely hostile with not really the "option" of developing- more like areas to avoid. Most people didn't travel further than their village. The middle class was barely emerging- most people were serfs belonging to the land. The world was a different place 500 years ago and the idea of restrainting development of the "savage wilds" would have been a pretty insane/alien concept.
Of course, it would depend on what people you were talking to- hunter gather groups would have been in the "wild lands" and probably saw a shortage of land in which they could use- they would want more land that they could use, barring them from using land would have seen as something hostile.