The cartoon was childish, so that's not surprising.
The point is that Ock was a major threat on every level. He had his own goals and schemes and the power and intelligence to act on them. Same with Osborn.
They were not as Venom is, a one trick pony out for simplistic revenge for something pretty stupid.
All Venom can do, would do, is fight Spider-Man. And as Spidey 3 and the early comics show, rather easily dispatched. Ock and the Goblin were threats not just to Spidey, but to the public at large. Same with Sandman. Venom doesn't fit the main villain mold.
Ock potentiall is BUT i don't believe he was shown to be in the last film. Not properly. I don't think he outsmarted spidey in any shape or form, out fought him, or showed a greater intelligence (with parker making clear observations beforehand on how his work would be unstable).
No need to diss cartoon ock, in armed and dangerous, he didn't come across as childish. He was arrogant, intelligent (trying to trick parker into mentioning the wrong paper), hot tempered, highly skilled (taking spidey out with only two tentacles), having an abilty to negotiate and put pressure on his foe (while he was stuck on the electro magnet he forced spidey's hand to let him go) and was quite clearly work obessed shown by his own private research as well as his inability to see how his fusion battery could not exceed the small size (which he at least managed to do unlike the movie ock).
those are all my observations, maybe he did exaggerate some of these points and if that's what you mean by childish so be it.
Yes you're right venom is a one trick pony but he does that trick well enough to hold a single movie...
Are you trying to say that all of this couldn't fit into a film
parker finding the symbiote
parker going bad
meeting with conner
parker losing the symbiote
venom's first encounter with parker
venom going to jail
venom getting out
venom taunting parker at bugle
venom taunting aunt may
venom pushing parker too far (like when he spilt offel on him in the comics)
venom and spidey's last fight at a construction site when parker finally gives in and allows the symbiote to try and take over him
this pretty much is an almagam of venom's first 3 or four comic stories with the inclusion of some aspects of the animated beginning for simplicity.
it kinda works for a trade paperback so i think it could work for a film. True only one film but it could still fit.
also it would cover aspects not really shown before such as a villain esaping custody and getting out again to come back after spidey.
i know their 'might' need to be some minor villain in there as a reference point but it would be venom's film entirely and i believe for a one off, it coule work.
EDIT:I haven't even mentioned some of his anti-heroic behaviour that could also be exploited in the plot to show variations of how he does things to how spidey does things and the parallels and differences in their approach and thought processes.