What is this software? E.g. Linux is 33 years old; barely a few percent of Linux 1.0 remains in a modern kernel, if we count lines of code.

Maybe GNU Emacs has a larger percentage remaining intact; at least it retains some architectural idiosyncrasies from 1980s.

As of Fortran, modern Fortran is a pretty nice and rich language, very unlike the Fortran-77 I wrote at high school.