It is eternal, as long as you have the system and the software that it was created with.
This can be said for all sorts of things, from the Commodore 64 to Keynote. If you want to be able to access the data on an NES cartridge you are going to need special hardware and software. Yes the ROMS are available now online, but that's only because someone with the hardware and software made it available to others.
Indeed. When you choose to save your work in a not-public format, you are decidîng not to care about its lifespan: complaining later is just useless, unfortunately.
This can be said for all sorts of things, from the Commodore 64 to Keynote. If you want to be able to access the data on an NES cartridge you are going to need special hardware and software. Yes the ROMS are available now online, but that's only because someone with the hardware and software made it available to others.
This is the nature of proprietary formats.