Well a large number of problems facing humanity can be dismissed as "just price in externalities/reduce transaction costs and everything will be fine" but until that's achieved tactics for reducing the social costs must be considered. I do wonder how many of the people responding to a given complaint with "Externalities!" actually go on to at the very least think about and discuss those external effects, and how many respond with the mentalese equivalent of "that's that problem sorted". This example is particularly instructive, since it's unlikely anyone on this board is in a position to make major reforms with respect to land tenure and property rights.