Re-writing these systems in another language is almost not an option. They have been written over decades with countless undocumented projects. They are core systems that work the way the business wants (or is built around, for better or worse), and doing a port would take years 100s of millions budgets and have a very high risk of failure.
I've seem smaller/non-core mainframe systems ported and they have been years delayed and plagued with issues, where it is very difficult to specify what is wanted other than something that is the same as the systems that is currently there with all it's weird corner cases.