I have to say, TF2 is one of only a very FEW games I continuously come back to and play, over and over again. I never really get bored with it. Maybe I'm in the minority, but I get the idea an awful lot of people abandoned that game simply because they got their attention re-focused on the latest and greatest, shiny new releases.
But IMO, Team Fortress 2 checks all the boxes for a truly fun gaming experience. The 3D shooter category only has so many basic concepts for multiplayer play, anyway. You have your "Capture the Flag" mode, your "Deathmatch", your "Domination", and so on. The only thing that really changes when you pay your $50 for the latest one are the background scenery, design of the levels and the characters. You get some unique weapons too, once in a while, but even those are usually re-hashes of the same ideas 90% of the time.
To me, TF2 ensures plenty of people to play against at any time, because they gave the game away free and it runs well not only on multiple platforms, but has reasonable hardware requirements so even older machines can run it. The bandwidth usage isn't too bad either, so your people stuck on say, a 3mbit DSL connection, can still play it without issues.
I'd rather see more development on top of something great like TF2 than wasted "reinventing the wheel" on yet another FPS title that will just get played for a little while and scrapped.....
(The best thing Valve could do for TF2 though, IMO, is to release some more official levels for it. I've played a few really good custom made ones, but also a lot of buggy, not so well thought out ones that even crashed the game at times. It definitely doesn't have NEAR the enthusiastic level building community that we had for older classics like Quake. It could use an official "level pack" or two from Valve, even if they cost a few bucks to download.)