The worst-written software always lives the longest. Maintainable, modular software, by definition, is easy to change and swap out. So the evolutionary forces favor awful spaghetti codebases that everyone is too afraid to touch.

So then the solution would be true natural selection and self-evolving software. As long as external entities (programmers) are tasked with killing off the unfit code, selection is going to be pretty bad.

Nah, they weren't buggy, it was just user error. Nobody stepped on them properly.

I can say that emacs will survive till the end of the keyboard era. It's not tied to any OS, it's portable and is a general purpose programmable programmer's tool. It's the archetype of long living software already.

Most likely some military control systems will live much longer than the designers thought...

DMV's systems (given their speed) are from 80 years ago

So that's a scary thought... but what's an even scarier thought is that they were probably made pretty recently.