It just would be a very time consuming experiment to run, and the cost of being wrong would be high, but I don't see how it isn't possible to get empirical results about if being a poet would, in fact, make you happy.
It would be easy enough to show that becoming a poet didn't make someone happy. But if they became a poet, and then became happy? Surely its pretty obvious that the connection wouldn't be testable in the slightest.
Actually, the success of science is its ability to predict what will occur in a wide variety of situations. Otherwise science wouldn't have been such a big deal.
If it were possible to specify how much happiness is enough, and how much happiness isn't, precisely, then it would be science. That's the only way to make generalizations from the results. But the threshold of happiness is imprecise, so therefore it's not science.
It just would be a very time consuming experiment to run, and the cost of being wrong would be high, but I don't see how it isn't possible to get empirical results about if being a poet would, in fact, make you happy.