Yes, but the idea is that even if the original author dies, people can still keep using the last release of the application, but independently write new plugins to interpret new image formats that come out in the future.

Winamp has a similar property. If in 2030 FLAC is replaced by some fancy neural-network based lossless encoder, it can still read the files and play them without touching the core of the application, because all that's needed is a single plugin.

Well IrfanView is open source, so if the original author dies I would expect someone else to pick it up.

Your description might help with one specific area (compression algorithms) but there are numerous other areas where you can't just use a plugin.