The fact that vegetarian and vegan diets are nutritionally sound diets for humans proves that our choice to eat meat is exactly that, a choice. When a person chooses to eat meat, they are choosing to kill, to impose unnecessary suffering upon non-human animals.
The belief that humans and animals are equal is not necessary for the belief that ethical veganism is superior to omnivorism. All that’s required is a belief that animals and humans are equivalently entitled to exist without externally imposed unnecessary suffering, not that they’re actually, technically equal.
One can take the belief further, if they wish, as I do. But they don’t have to. The simple belief that non-human animals are entitled to exist without externally imposed unnecessary suffering is sufficient for ethical veganism.
This is merely sufficient. It is not a complete justification for veganism, but rather just the beginning of the discussion. For, there may well come a day when animals can be slaughtered entirely painlessly. When such a day comes, veganism is still the appropriate moral choice, but for other reasons that simple suffering.
But for now, veganism can be simply and easily justified through a desire to end the suffering of animals.