Why exactly is 2006 the dividing line? Cartoon Network at that point was unrecognizable compared to 1992...
As someone who remembers when we first got Cartoon Network in 1996, 2003 was the start of the end.
The golden years were 1998 - 2002. I say this based on the balance of quality kid's originals, movies, anime and action shows, forays into adult cartoons, and showcasing the prior 60 years of animation history in an accessible way to younger viewers.
2001 and 2002 started to have issues but I would argue the rot really set in in 2003.
The first obvious signs of going downhill came with the weekday expansion of AS at the start of 2003 I'd wager. I liked Adult Swim but Futurama and Family Guy set them down on a path from which they would never return. Plus the shows just got a lot more edgy, and not in a good way (at least in terms of relation to Cartoon Network; I do enjoy some later Adult Swim stuff).
An early warning sign prior to all this was in 2000 when Boomerang was launched as a separate channel and slowly, every few months the lesser known stuff started to shrink from the Cartoon Network lineup only to reappear over there. Only Scooby Doo really remained intact a CN staple. Then, circa late 2001 CN started phasing out programing that targeted older viewers en masse assuming that the new Adult Swim was enough to handle those viewer at the time. They also removed Sailor Moon, only to replace it with endless Dragon Ball. It had started a trend.
By the time of the AS expansion, CN started taking a lot less risks and throwing out classic material (ie. slotting all the pre-90s Hanna-Barbera to Boomerang and canning Toonheads and the Chuck/Tex/Early Black and White MGM showcases).
Circa 2003 and 2004, a lot of the good, new stuff got canned (Robot Jones pre-human voice, Sheep in the Big City, Mike Lu and Og, et cetera) and anime basically took over Toonami, which moved to Saturdays.
Plus, no more Total Immersion Events for Toonami.