I personally can't decide a date but here are all the crionicles of nick's downfall:
Late 1994: Doug is canceled/ended.
Mid 1996: Doug is sold to disney but sadly but truely the disney doug was more popular in fact it was so popular it got books and a movie.
November 1996: Rocko and Ren and Stimpy and both canceled/ended.
Early 1998: Slaut your shorts no longer aired reruns.
March 1999: Nick screws up All That by firing the most talented actors to put the most no talent actors on this show in fact the actors they hired in 1999 are so untalented that they should of canceled it 7 years ago yet they waited till 05 to cancel it.
Late 1999: All of the old nick shows like Rocko, Ren and Stimpy, Real Monsters, Old All That eppys, Family Double Dare, GUTS, Wild and Crazy kids, Tiny Toons, Looney Tunes, Garfield, Nick Arcade, Legends of the hidden temple and Inspector Gadget disappear without notice.
Mid 2000: Are you afraid of the dark is removed.
April 2000: Kenan and Krl is canceled/ended.
November 2000: Nick announced that Catdog was gonna be discontinued.
March 2001: The Angry Beavers gets canceled.
Late 2001: Catdog is truely canceled.
Mid 2002: Invader Zim get canceled meanwhile it still had potentially strong ratings and worldwide fan popularity.
Summer 2003: The overdosing on current nicktoons becomes a bad habit.
Early 2004: All That get screwed up more than it already has.
Fall 2004: Unfabulous makes it's way to nick.
Feburary 2005: Nickelodeon starts to jump the shark more than it already has.
Feburary 2005: Nick announed nicktoons is gonna become ad-supported.
September 23 2005: Nicktoons becomes Nicktoons Network and it gets an ugly new look and all the crappy nicktoons like My dad the rockstar and Martain Mystery make there way over to nicktoons.
November 2005: Nick GAS gets rid of pumping gas, scardey camp and all it's other fine programming to make room for repetetivity.
Janurary 2006: All of the classics nicktoons such as Rocket Power, Action League NOW, Real Monsters, The Wild Thornberries, Doug and mostly all the other classics still remaining get replaced with addition repeats of Martain Mystery, My dad the rockstar and Kaput and Zosky.
And beyond Jan 2006 doesn't look too good it seems nicktoons network is gonna have origina; shows.