Actually, while ACP flopping as hard as it did was a horrible mortal blow, the real killing strokes that doomed Spike's adult animation block were two-fold:
1. They didn't go hard in for anime, especially older shows like say, Ramna 1/2 or other edgy animes that Cartoon Network/Adult Swim wouldn't touch with a ten foot poll. Anime was the backbone of Cartoon Network going after older viewers and Spike didn't even try and tap into that market
2. MTV cockblocked and cockblocked hard, Spike in terms of not allowing them near their animation archives. The only thing MTV let them have was a random selection of hacked together Beavis and Butthead episodes minus the music video segments. Daria, which had just ended and would have been the perfect counterbalance/flagship show along with Ren and Stimpy, wasn't offered to Spike as MTV instead syndicated it to Noggin (which is what became Teen Nick), where it was butchered and dropped after a couple of months. Nor were any of the other MTV originals: no Celebrity Deathmatch, no Aeon Flux, no Liquid Television/Cartoon Sushi, no Max or The Head, etc. IE shows that had a huge fanbase that could have created the backbone of the Spike cartoon block.