I agree as far as the feature set is concerned, but software unfortunately doesn't exist in a vacuum.

A vulnerability could be discovered that needs a fix.

The operating system could change in such a way that eventually leads to the software not functioning on later versions.

The encryption algorithms supported by the server could be updated, rendering the client unable to connect.

It might be a really long time before any of that happens, but without a maintainer, that could be the end.

That can be true for self-contained command line tools, but not for complex programs with actively development dependencies (especially anything dealing with networking or encryption). For example hexchat uses GTK2 which is likely to be removed from mainstream distro repos in the coming years because it has been obsolete for a long time. Also openssl which is known to change its API occasionally which means that anything that uses it needs to be updated to stay compatible.

This. Sometimes a software is just finished. IRC itself has not seen change in like... about all the time I remember.