Changing Iajutsu from a cast time to instant does almost nothing because you still have to deal with the fact it is on the GCD anyways. Not to mention that after 70% of the cast, which is not even a full second, you get protection from interrupts allowing you to move and complete the hit regardless. Also removing the cast time removes flavor and thematic elements from the class. I would rather not see a class lose that after what AST went through.