Actually, in the case of Heavensward when they did bring in a new tank and healer, that was the case for maybe a month before queue times went back to normal. What had been more noticeable was that the vast majority of Dark Knight and Astrologian mains in Heavensward were already tank and healer mains before those jobs were released.
This is another thing that just won't happen because their argument that the tradition of Final Fantasy games having predominantly had a party of four will always outweigh any opinion players will have on the matter. However, I do agree that changes should be made to tanks and healers to make them feel more rewarding, but I also feel that it should come with making mobs and bosses deal more damage to non-tanks.