I’d already be very happy if everyone took your approach, but it’s not the entire story for veganism. Sustainability is an important factor for myself and many others, but so is animal welfare.
It’s a bummer that animal welfare is pretty much inversely correlated with emissions. Packing chickens together and making their lives miserable is much better for the environment than having them roam free.
Veganism happily aligns with environmental sustainability. But when you believe we shouldn’t exploit animals at all, just pushing to eat what’s sustainable ignores a lot of pain and cruelty.
This is a false equivalence; the answer is “neither”.
Veganism doesn’t seek to end all animal suffering, but not to exploit animals for humans’ sake. We don’t need animal products to survive, so we shouldn’t add to whatever misery already exists naturally.
In the case of livestock, we should just stop breeding them. No vegan is arguing for dumping all cattle in the savannah to be hunted by lions.