How can we determine whether there are enough people doing the stuff already? Going back to my comedian example, should I get to be a comedian just because I started doing it at some minimum acceptable quality, before there were 'enough' comedians, whilst the next Louis CK has to do something else?

I think you're missing:

- we don't need to measure productivity in aggregate (and I agree GDP has problems in doing this) but for each individual, and probably only in a relative sense

- competence in a field and motivation to do it are not necessarily correlated, as evidenced by the popularity of career-change guides

- A democratic process for deciding whether your work is useful for society would require a lot of information on each person's work. I don't see how you could summarise that information without coming up with a metric which means something like 'productivity'

Anyway, we've veered far off topic and I don't think we'll convince each other. Have a great weekend!