You are correct. There was a time when companies ran projects to replace their mainframes. They all largely failed. The trend now is to integrate around the mainframe. Build new functionality on modern systems and leverage the mainframe where it makes sense.
I worked at one company that is trying to replace their mainframe. The project is now in its second decade. The company has the burden of supporting the mainframe application and now the 'modern' application. The project is still a few years away from being able to completely shut off the mainframe.
Oh and the modern application is also showing its age as the business has changed and the modern system built assumption in from the mainframe system and in many ways is just as inflexible to change as the mainframe app was.
This is why companies aren't scrambling to replace their mainframe systems. They are happy to continue paying for the high cost of compute on these systems because its still cheaper and less risky than replacing the system.
