I've watched this happen to two of my favourite games. Both of them being Sierra games to top it off.
Empire Earth just got shut down recently, and the community is fading fast because of it. Thankfully, some people quickly got an alternate lobby system setup and existing players are still around, but new blood is effectively cut off and the game will die.
The other, and my personal favourite game of all time, is Starsiege: Tribes. This game hit the cycle too. Tactics were refined (disk fighting, hopping, and "skiing", which is like bunny hopping but downhill, and then jetpacking on the upside of the valley, essentially catapulting you into the air at great speed), the same maps started to get played, like Broadside and Raindance, and then the skill gap was created, causing newbs to get raped hard by even average players. The servers for this game are down as well, though it doesn't matter, because it doesn't run on vista anyway.