That's not really a reason with a business case attached. That's just "these other languages are modern, use them instead!".

No, it is the other way around and it surely has a business case.

Instead of throwing out the investment done in years in the mainframe hardware, because juniors cannot grasp old technologies or don't feel like using them, bring them into the mainframe, while keeping the investment into the existing working stack.

Instead of writing a REST API in RPG, which doesn't know anything about Web APIs, used to create the customer support application, make use of JEE/Spring instead to provide a SOAP/REST API.

Just a possible business case, that is actually used in production for Java applications on IBM mainframes.

There is nothing especially modern about C, C++, and Java.

C for sure isn't modern, never was, given ESPOL and NEWP in the 60's.

But it surely feels modern when compared against COBOL and RPG.

On the other hand, C++ and Java are quite modern for the overall mainframe mindset.