[waynecolvin]

Languages/libraries evolve and bitrot is a fact of life, yes.

Now you have me thinking about how to plan that long! You would need an architecture easy to port and good documentation...

https://mcnp.lanl.gov/

Or become so important that old platforms are maintained for your sake!

[adrianN]

Look for a job programming planes or trains or other infrastructure. Typical service time is ~30 years, some systems live much longer.