TF2 is an impressively well-aged game all things considered. it is, by now, appraoching it's 15th year of continually being out, a feat I can think of few non-MMO games can compete with. Part of that is off the back of really solid design from the beginning, along with an art direciton which would not degrade as contemporary technology improves. The writing helps a lot as well.
But, at the end of the day, I think it's tf2's community that has kept it around this wrong. The text chat filled with a mix of trans girls with anime pfps to raging bigots is sometimes tough to read, but it's never dull. It's something of that 4chan anoninmity factor, that ethereal idea that having no identity tied to what you say, you can say anything. Sometimes this results in games with revolting chats, but sometimes, most times, even, it's something faintly magical. A wave from across the screen. Nothing serious, nothing dramatic, and almost never wholly authentic, but something real.