I don't know about that. I've pretty continuously been playing games like UT99, UT2k4, Quake (specifically Quake Fortress), TFC, AvP2 since release. Even things like Space Station 13 and King Arthur's Gold.

It's not the only thing going anymore, but also people have orders of magnitude more choices. That's the thing that's missing today. Nothing is "dead", there's just too much competition now.

Honestly the aforementioned games have had much more staying power than more recent multiplayer games. And they're mostly played by old men like me.