The belief that the Cartoon Network (CN) Powerhouse Era started wrapping up in 2004 is popular, although I'd argue 2003 was when CN Powerhouse Era started to wind down operations. Of course the changes weren't fully felt until June 14, 2004, when CN Powerhouse era ended and their City Era began. 2003 already showed red flags of the CN Powerhouse Era going away.
Why I believe CN Powerhouse Era started closing in 2003:-Because CN began to gravitate towards original programming, the Powerhouse bumpers were used less frequently, "with certain shows only airing in programming blocks that used their own bumpers."
-When Adult Swim started to be shown on every weeknight except Friday and Saturday instead of just Sunday and Thursday weeknights, the black midnight-hour bumpers got discontinued.
-Newer shows that aired like Baby Looney Tunes, Duck Dodgers, Teen Titans, didn't have their own Powerhouse-styled bumpers.
-Powerhouse music was replaced with background music from the shows.
-Instead of shows having multiple pattern bumpers, that was cut down to a single pattern.
-Blue and purple were the only background colors instead of green, yellow and dark blue.
-Shows like Dexter's Laboratory, Time Squad, Grim Evil, Whatever Happened to Robot Jones ended.