You are pf course correct, but that was a different time. In today’s world, with the dangers, especially with threat of radical Islamic terrorists and their allies. The need for special landing craft is apparent. I am not saying that the LHA-6 is not useful. But that it has major flaws do to its lack of a landing deck.
And other amphibious platforms don't have USS America's ability to land large numbers of troops behind the beach as rapidly. Different platforms fill different functions. Let's face it. In the event of an amphibious assault the U.S. is not going to commit a single battalion landing team. They are going to move large numbers of troops on multiple amphibious ships, some with landing docks and some without. The America will operate as part of that task force.