There are still people playing Quake. Now, there aren't many people, but it's been 20 years- how many other games boast that? IMO, TF2 is on Quake's level, and it's gonna depend on what you mean by "dead". If Valve moves on from TF2 and develops TF3 or something, the playerbase will die off, but I imagine there will be a core group of dedicated players maintaining the abandoned game for many years after that deadline.