As time goes on, more and more people will be reaching their 50's and end game. More and more people will be doing the same dungeons repeatedly, and there will inevitably be more and more lag. SE may be in part to blame, but the fact that more people are reaching end-game is just that, a fact. It's also a fact that lots of people in one area causes slowdown.

If people want to run the same content over and over again, that's their prerogative.