What social metric in particular? Also, for better or worse, social metrics are easily gamed. While a basket of goods can also be gamed, it's easier to see what's going on and to explain it to people, since it's composed of concrete goods. Also, a basket of goods can be expanded if we want to increase the baseline as living standards in society improve, which we do - a smartphone should now be part of that basket, even though they barely existed 20 years ago.