Once it stops getting software updates, its life is effectively over- that long-life PC's life ended years ago. People keep going with old hardware for different reasons.
If they enjoy the challenge of maintaining old machines past their expiration date and don't mind dealing with the annoyances, then that is a total geek thing for sure (not that there's anything wrong with that). People doing this understand and accept the challenges of doing this, and usually don't go around complaining about it.
If they can't afford a replacement, there's always the "just put Linux on there" option, but it's unfortunate that they can't just get a new machine. This rarely seems to be the case in these discussions though, at least on HN.
The most common non-nerd reason for clinging to unsupported hardware seems to be self-defeating stubbornness or cheapness, where the person feels a compulsion to keep it going "for the principle of the thing", where it's almost like they're doing it out of spite. If they want to do this to themselves, fine, but it doesn't make it any less self-inflicted.