There are really applications that are large enough and hard enough to parallelize and/or shard that any rewrite on a different platform would turn into a performance catastrophe, even if you hired the best engineers and wrote the whole thing in very efficient C++, which you never hear about them doing because its usually only about saving money and so it's done with as cheap of developers as possible and in Java. I've seen it and it's not pretty. They tend to be dog slow and unresponsive, and even harder to maintain than the crusty old assembler and COBOL, because you have to implement a lot of the report writing and record crunching features built into a domain specific language like COBOL from scratch it you want to write the same application in Java.

That's my biggest pet peeve with people that want to ditch mainframes, which is that they seem to care very little about the quality and performance of the software in my experience or they would only be thinking of replacing COBOL and Assembler code with an equivalently performant modern language and dialect. The desire to migrate is often driven primarily to have cheap, easily replaceable developers.