And it might also be one of the best measures of aggregate economic activity for a large region, which could also be considered a good measure of............?

defining well-being, and then measuring it, is as pointless as defining happiness, and then measuring that. happiness is a notoriously unreliable indicator of anything. it's based on an individual's perception of his/her state. there are cultural differences. there are differences by station in life. and worst of all, there are differences per individual based on everything from the season to their age to what they ate, read or did 5 minutes prior to measurement.

i use infant mortality as an objective measure of any society's success. that's because everyone in the world can agree that dead babies are a bad thing. and the data is objective and easy to measure. it is, in a sense, a better indicator than GDP. the recent example of China is probably the best example of this. For decades after the cultural revolution, China was able to decrease infant mortality, without similar increase in GDP per capita (which came later).

That would be because GDP is not a measure of collective well-being.

Can you back this statement up, please?