The "rot" is because the environment is changing (the hardware you're running it on, other software you need to interact with) or the software is being carelessly updated.

If you need to, software rot can be eliminated. It takes effort but it can be done.

To give an example: you can run a space station with software installed in the 1980s, but you probably can't run a space station with seals or pumps installed in the 1980s.