Columbus would not have ventured forth if he thought the waves got bigger the longer you were out and away. The morning traffic jam is ultimately constrained by the number of cars and the geography of the road network.

If you look at outputs, and use dynamic models (which Keynes could sense acutely (he referred to “animal spirits)), and your models don’t converge over the relevant (to you) timeframe, it is hairier, and you can invoke chaos theory. But even then there are things that can be done. Canoes get through the rapids better if canoeists keep their hands on their paddles, and their paddles in the water. There are many butterflies in Central Park, and big Mr Sun working on El Nino out in the Pacific will easily trump a great flickering flock of them, and in no case will we get thousand mile per hour winds, nor glass smooth oceans. So partial analysis can be useful.

But your larger, unexpurgated quote is interesting. I think Keynes was thinking more about the “life spans” of consulting or advice-giving economists, which is usually shorter than the cycle time of the economic phenomena that they are attempting to study and prognosticate about. (So they don’t have time both to learn and to reapply their learning, and make it better).

Apparently Napoleon’s last interview question before he hired a new field marshal was “and is he lucky?”, and I suppose that businesses and governments still ask that, notionally, of their analysts and advisors.

Governments sometimes outlive economic advisors, and sometimes they die from them.