You make some great points that measuring could be improved. Never the less so long as their method remains consistent, it shows the world getting richer or poorer relatively speaking.
For all the antiglobalization sentiment, I can't imagine this happening without globalization. If the workers of the developed world had put an economic moat around themselves and preserved their trend in rising purchasing power and had wages increase undeterred by labor competition from both shipping jobs overseas and allowing mass immigration of unskilled labor, we'd not have seen these improvements and those places, given the pop growth they experience over the last sixty years, would have been severely more impoverished.
For all the antiglobalization sentiment, I can't imagine this happening without globalization. If the workers of the developed world had put an economic moat around themselves and preserved their trend in rising purchasing power and had wages increase undeterred by labor competition from both shipping jobs overseas and allowing mass immigration of unskilled labor, we'd not have seen these improvements and those places, given the pop growth they experience over the last sixty years, would have been severely more impoverished.