When you have new type of A/Cs in the company, you also add employees to fly and maintain the A/Cs. And you probably have different teams to deal with different type of A/Cs. For example, you will have one team, including the pilots and engineers, for A343 and another team for B747 without overlaps. I believe that pilots who fly A343 will not fly B747 because the operating systems are different. To learn more systems just make them easy to confused and increase the chance to make mistakes. I believe this is why we have aircraft commodity.
Moreover, when you add a B744 to your pure A343 fleet, you will add a new team to fly and maintain the new B744 not training recent team to fly and maintain the new B744. This should not affect the cost of original A343 team. Maybe employees in logistic and warehouse have to train to manage the additional component, but the cost should be little.