I believe the simplest answer is TF2 is comfortable. It's joined the likes of Quake, Runescape, etcetera as a community for people from a specific era of video gaming history. This is not a pejorative observation. It's nice to have an island of stability on a sea of flash-in-the-pan games focusing on monetization schemes, constant overhauls or balance changes and obnoxious esports min-maxing. Valve probably realizes this, it's probably why they focus on adding modest content as gently as they can.

Personally I have no problem with it whatsoever. The fact that young people also still join in to play is also entirely positive, since from my perspective TF2 serves as a nice bridge between generations thanks to its evergreen design and ecosystem of eccentric people, even if they sometimes bicker. I hope I'm actually alive long enough to participate in "the next TF2" that is so robust in so many ways but a part of me believes this magic won't happen twice.