> An honest question: If you are writing a program that you want to survive for 100+ years, shouldn't you specifically target a well-maintained and well-documented VM that has backward compatibility as a top priority? What other options are there?

I'd be tempted to target a well-known physical machine - build a bootable image of some sort as a unikernel - although in the age of VMWare etc. there's not a huge difference.

IMO the "right" way to do this would be to endow an institution to keep the program running, including keeping it updated to the "live" version of the language it's writen in, or even porting it between languages as and when that becomes necessary.