I work for one of the major banks and all the reasons you listed are true. From talking to coworkers who work with the mainframes, the mainframes still provide fantastic performance that Java and other higher level languages haven't matched on the stack (yet). Also, the applications are so large and so critical to our customers that the process of transitioning to a new stack would be a nightmare for almost zero short-term gain.

It should be noted that the industry is trying to make a push for porting the COBOL applications to Java to help resolve issues regarding an aging workforce, hiring, and documentation.