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Batch job control is handled by alternative systems (already often triggering the work on mainframes anyway) like Control-M

CICS is the big issue but AFAIK there were attempts. Everything else, including sort and VSAM, has various options provided usually by COBOL compiler vendors.

If the systems are to stay on mainframe/micro land, all IBM, Fujitsu, Unisys platforms support Java.

This is one set of ecosystems, where .NET going cross platform still misses out to Java, and neither of those companies cares about supporting it on their platforms.

There are some half way supported Python, Go and Rust versions as well, although I think they need the "WSL" of the respective mainframe/micro OS, aka UNIX services.

For the article they just mocked it out for unit tests. But in reality you really can't - that’s a massive pain in the ass. Rewriting pure math is easy, but mocking CICS transaction isolation in java means spinning up these monstrous adapter frameworks that just tank performance