"old code runs fine" is a poor benchmark for how well a language holds up over time. All x86 code still runs fine. Perl 5 is from, I think, 1994. It's still maintained and in development. Most C code from the '80s would still compile and run just fine (at least POSIX-based, Windows however...). You can trivially find Pascal, Fortran, COBOL, and Forth code that all "still runs fine."
I don't think it's a particularly unique or interesting quality, that some old code still runs. After all, I can go to archive.org right now and run all of that ancient DOS, Amiga, whatever code in a 100% exact (or close to) emulator in my browser.
I don't think it's a particularly unique or interesting quality, that some old code still runs. After all, I can go to archive.org right now and run all of that ancient DOS, Amiga, whatever code in a 100% exact (or close to) emulator in my browser.