> And what's the oldest software that's still being used currently?

I am very interested in this question, but much of software is proprietary and you don't have much visibility into their history.

For software in the public, trivial lower bound is GCC 1.0 in 1987. Another case I know is Community Climate System Model, in continued development since 1983.

The oldest software still used is operated by banks, government agencies, schools and businesses. A large number of them are black boxes, and very expensive, so nobody will replace them. Their hardware just gets maintained ad infinitum, and the software doesn't care how old it is, so it just keeps running.

So basically, the age of software is limited by its hardware. (It's similar with animals: if nobody bumps them off and the hardware keeps ticking, the software probably will too... fish and mammals can live for hundreds of years)

My favorite examples:

https://www.pcworld.com/article/249951/if-it-aint-broke-dont...

This is way more than a thought experiment. Stable, mature software runs the world.

E.g. the CIP project aims to maintain released kernels for 25 years. https://www.cip-project.org/

Unix. Email. TCP/IP. Just taking an initial stab.

All of those will be around not because they are good, but because so many other things are built against their interfaces.

Ultimately, what else is 'good' other than being the foundation of further far-flung flourishing?

Those are standards and protocols, not software.

Spider Solitaire

I would place money on people still using Vim and Emacs in a hundred years.

`ed`, of course

Git.