Green Armadillo has a good post on Scott "Lum the Mad" Jennings' talk about the lifecycle of the MMO player.
I think I'm with Green Armadillo in not really understanding what Jennings is trying to get at. Yes, people will get tired of your game and move on. But I'm not sure what advice he is offering the developers. Is it just emotional advice? Don't take it to heart when a player quits after several years? Assume your MMO will die after 4-5 years and plan accordingly?
The only other interpretation I can make is that Jennings is telling devs to avoid doing excessive work, or avoid trying for "Jesus features", which you think will keep people playing.
Maybe he is trying to say that an MMO developer should concentrate on their core gameplay. For example, if your game is about PvE group content like dungeons and raids, maybe you should stick to dungeons and raids, even if you know that people will eventually tire of it and move on. Maybe he's saying that you should not spend time and effort developing new gameplay modes like Pet Battles or Galactic Starfighter to reignite a player interest.
I don't know if that is what Scott Jennings is saying. I don't know if avoiding new gameplay modes is a good idea or not. Or if Jennings is trying to get at something completely different.
If I could give gamers, programmers, and developers one piece of advice when it comes to talks and posts, it is to be blunt and obvious. Assume that we the audience are stupid and hit us over the head with your thesis. Don't allude to it or try to be clever.