I was thinking on Venom and originally he was supposed to be the anti-Spider-Man, and one of his top villains. Because in a way he was the dark side of Spider-Man. A villain who was not responsible or cared for others, he only cared for his own needs.

Later on Carnage comes along and has a similar symbiote costume. I think right there, having another symbiote by default diluted the Venom concept.

Then to make him a killer already, before he got the symbiote, and making this version even stronger then Venom and Spider-Man, as shown in past comics.

I think overall Carnage, took over and replaced Venom as a top Spider-Man villain. And if anything it diluted the Venom concept.

At least other symbiotes that were made did not last. Remember the 6 villains version, Hybrid or even Toxin? All further imitations on the same basic concept.