[PeCaN]

The plus side is that modern mainframes are really cool. It's not like they're still systems from the 70s, they are practically supercomputers with some of the fastest hardware ever made. (zEC12 was a 6-core, 5.5Ghz processor with very competitive IPC and monstrous cache; I believe it even had functional transactional memory too coughIntelcough. Now stuff 20 of those into a computer.)

Downside of course is COBOL, Java, etc. But it's not like it's going anywhere soon. Nobody really wants to replace millions of lines of COBOL that work, and work well.

Writing a compiler for an ML-ish language that targets COBOL would be kind of amusing. Just because they're stuck with COBOL doesn't mean it has to be hand-written, right?

Disclaimer: I don't work with mainframes but I know people who do. Friend of mine quite likes writing assembly for z/Architecture. I don't know if she's totally sane.