There are few things that would make me think so.
The first would be if there weren't any people working on the engine (modifying it, mapping, modelling etc..). Like the whole hl mapping business would freeze, no new content, nothing. This would mean that it's dying like from inside. Of course you don't need new content to keep it alive, there's enough of it already, but.. new content is like.. fresh air...
The second, and in my opinion the most important would be if there were no people who would still enjoy playing it. If i would not see any hl engine servers, and if all people i know would stop playing it. I sure have great time playing hl dm with my friends on lan, it would be a huge loss if noone would play it anymore. But i don't think that's possible.
I'm optimistic, and i think that hl won't dye that easilly, even if there were a lot less people working on it and playing it. Like Ant said, as long as there are people still playing/modifying it, it's alive. In my opinion there will allways be people who play or modify it so it's almost impossible for it to die
The only time it will die is when power supplies for our electrical equipment diminishes/ends.....but I will be long gone by then
True.
Edit:
Heh, legends don't die that easilly