An Interesting Theory (For Those Who've Seen It)
So the gargoyles spawn every 500 Earth years to wrestle control of the mortal realm away from humanity, appearing in all parts of the world, various cultures and folk tales from the sands of ancient Egypt to the castles of medieval Europe. Are they really necessary in destroying the human race considering all the deadly, destructive conflicts we put ourselves through (The World Wars, Black Plague, nuclear annihilation, and so on)?
I know this is just a made-for-TV movie and am putting too much stock into it but my point is: even though the gargoyles are evil in their purpose and nature, they serve as a unifying catalyst in bringing all of humanity together to fight a common enemy. In this sense, they stop humanity from destroying themselves. Assuming the movie takes place the year it was filmed, 1972, that means these hell bound creatures last surfaced in the late 1400s, shortly before Columbus's historic journey to the new world, ushering in unprecedented contact in the time of man. It may explain how the gargoyles gradually ended up in what would be the southwest of the continental United States.
Last thing to consider. The gargoyles were defeated before the next generation could be born from their incubation period by a small band of people with only a handful of guns, gasoline, and a lighter. Can you imagine what the gargoyles might face against humanity in the future, if we've managed to survive from imploding ourselves? The fate of humanity is uncertain and the ending should give us pause.