You're absolutely right. But to start a decades-long project that may or may not work and has no near-term ROI is a hard thing to rally for. The cost of the project would eclipse what is being paid to the mainframe provider and to company personnel. And these companies have a difficult time bringing in top talent, so they don't really have many tech-savvy people to preside over it.

I've seen it attempted first-hand and it was a dismal failure. And it was their third or fourth attempt. And it wasn't even a full replacement! Just a simple data lake to dump things into. Google or Facebook or Amazon could do it, but few companies are like them.