[madmulita]

I believe the real reason for its survivability is the fact that you can pull a tape from the seventies and those binaries will run without any modification. It's not only that you can easily recompile your COBOL from the '70s, the binary is still compatible. You've never been pushed to migrate to another technology. Imagine the effort and 'knowledge' included in those evolved programs. The banks don't even know, and are conscious of it, how many laws and regulations they have encoded in there.

As someone stated in another comment, the software is the impressive part.

[technofiend]

This is both good and bad. You have to consider bugs a kind of feature like anything else. One of my coworkers showed me a bug report he'd opened 30 years prior that IBM still refused to fix because people depended on the broken behavior. So porting off the mainframe also means bringing along those quirks or rewriting to specs that provably don't regress performance and behavior. Writing or rewriting software is easy, but migrations despite how they first appear are not really "green field" development.