Thats a very fair argument. But lets say hypothetically that language X was more "productive" than language Y. Wouldnt it be a worthwhile "investment" to learn X seriously? Sure it might slow you down for a year, but after that it pays dividends.
However when dealing with a team often with an existing code base getting everyone on board with "X" while potentially porting to "X" is fairly expensive. Its generally only worthwhile because of actual technical limitations and not productivity. Even the promise of safety (ie prevention of bugs) I would probably consider higher than productivity (although I suppose that is in way productivity).
Regardless productivity is really hard to measure and especially predict unlike other technical things.