People still play TF2 because it's a free to play game on Steam that isn't total garbage.
When I say it's dead, I don't mean nobody's playing it. I mean nobody's thinking about it anymore. Nobody takes it seriously anymore. Nobody really gives a shit about it these days, it's old news.
You can say it's still popular because it has like 60,000 players or whatever, but consider this: Counter-Strike Global Offensive ISN'T free-to-play, and it has 10 times as many players as TF2.