most living creatures are carnivores.
Actually that's not true. The heirarchy of predators and their prey is triangular in structure, meaning that there will always be more prey than there are predators, e.g. the population of fish in the ocean is higher than that of sharks and so on.
This naturally means that as we descend the food chain there will be more herbivores compared to carnivores, because there is a higher population of them.
Your theory doesn't cut it with me. Just becase a carnivore eats something, it doesn't mean the thing it eats is a herbivore. A carnivore can prey on other carnivores.
For example, you're saying that the number of fish in the ocean outnumbers sharks. That's correct. But a shark is only one kind of predatory fish. There are a lot more predatory species of fish in the sea. Baracuda, types of piranha, types of eel etc. Maybe one of those species could outnumber sharks, but a shark is a better predator so it will pwn anything smaller than itself.
I'd say the food chain is more to do with size and adaptation as oposed to food preferance. The more skilled/powerful the predator, the higher it is on that pyramid or whatever the fuck.