One of the things I DESPISE about modern Marvel, aside from the general watering down of their properties (I dry-heave when I see Thanos in any comic for any reason at this point, and I used to like the character) is this just... lack of forethought.
Venom was some goo that came out of a random machine on Battleworld in the mid 1980s, possessed Peter (IIRC isn't that why the symbiote is spider-themed?), did stuff, they fought, etc. Classic supervillain story, in a way.
But now thanks to Bendis and Cates and similar hacks literally EVERYTHING about the symbiotes has been retconned and altered beyond recognition. One day they're an evil race bent on galactic domination. Then they're the noble Klyntar, brainwashed or whatever, I barely skimmed that run. Then KNULL, evil dark god of dark evilness.
What ARE the symbiotes anymore? What's the thematic undercurrent? What do they signify, what purpose do they serve? If you think about it, any recurring character has to serve some purpose - Aunt May is Peter's family, she grounds him, she's the constant reminder of Ben and Peter's motivation. MJ is there to support him, to show him he's not alone, to be someone who can't fight beside him but she'll always BE beside him. Etc. Same for the bad guys - Ock is "what if Peter was raised by assholes"/"what if someone jaded and non-heroic got into a lab accident that gave him powers". Osborn is the nightmare, this fear that he can't keep his lives separate, that his enemies will learn who he is and find his loved ones and do horrible things out of revenge. And Venom is, well, evil Spider-Man.
The only reason characters survive for decades is if there's something archetypal about them, some storytelling function they serve well.