It's funny to see someone ask the question: 'Is Civ III endangered?'
Even the fact that this simple question has already led to 4 pages full of answers in the Civ III forum would lead to a simple conclusion: You're kidding!
Most games of the same age as Civ III would be jealous of a game that still generates so much interest after so many years.
I played Civ III intensively for two years, then thought I needed a change, and I haven't touched the game all year (hence my slow answer in this thread). I'm now playing Age of Wonders: Shadow Magic; also an entertaining game, but with a much smaller community than Civ III. Still, even a game like Age of Wonders will keep going as long as there's a small core of people devoted to it, so no worries for Civ III.
Civ III is still HUGE. Just look at the modding section: one guy is presenting a modest new unit and immediately twenty people go 'RHOARRRR!!!' like they've just seen the babe of the century walk past.
Works of art that are good don't die; just look at some earliest movies, comics, literature, whatever; when it's good, it remaines in the pubic conscience.
And yeah; Civ IV would stand less chance of survival than civ III, for reasons already cited.
By the way, Argetnyx (and off topic): I like you a lot better now that you're saying you're young. You've got an avatar that was a propaganda item for nazi Germany, and by having such an avatar you could be seen as someone who likes fascist ideas, but if you're young, then you're probably not too aware of this. In Europe, where I live, nazi symbols are only presented together with the line: 'those guys were really wrong'.