Every game gets old, every game reaches an end to each individual, in time. If you play too much, the game gets old.
I remember getting burned out numerous times during my many years of playing Diablo 2. I'd quit. I'd take a break. I'd play again and, inevitably, the thoughts of 'what to do now' always came back until I finally hung it up for good.
All there was to do is trade items back and forth, do some MF runs, gain some XP doing Baal runs. Maybe some PvP in between runs. After awhile, the shine wears off and so begins the rigor of realizing that all you're doing is repeating the same actions over and over again just to better prepare yourself to repeat the same actions over and over again.
Have fun, play your own way. Stop chasing cheaters and unemployed slackers. Stop predicating your Diablo 3 experience entirely upon your success on a dubious leaderboard.
4 years and I'm still having fun. I still come home and often sit my ass down, smoke a bowl and play Diablo 3 for a few (several) hours. 4 years now, and I gotta say, the game is more fun for me now than it ever was. I don't play as much as I used to, but when I do play, I have more fun nowadays than back when the game was lousy with underwhelming itemization and lackluster build pliability.