Since IBM z-Series just hit the frontpage, you can add lots of COBOL and PL/I code which runs unchanged for the last 50 years. However, it is all proprietary and impossible to estimate.
Thanks for the link. That is a rather fun read. Is interesting to read someone that has "mostly given up on the standard C library." I think I understand and agree with the reasoning, to be honest, but is still far far from what is recommended in academia. Or industry, for that matter.
The COBOL and related code is an interesting data point. I would be interested to know just how bug free it all is, versus just used in ways that don't trigger bugs. Still, they definitely count as long running software, if not highly ported.
Since IBM z-Series just hit the frontpage, you can add lots of COBOL and PL/I code which runs unchanged for the last 50 years. However, it is all proprietary and impossible to estimate.