Before humans intervened, there weren't so many cows, and other elements of the environment managed to offset their ecological harms.
Now it's not possible. You can't just grow as many cows as you want and say "well, in before times things were alright". They were precisely because there was less cows. The fact that methane doesn't stay in the atmosphere forever doesn't help, because at any given time, there's enough cows to maintain high levels of it. It's not that we keep on accumulating it for all eternity, it's that current levels are not sustainable and we keep on maintaining them (and you seemingly suggest we should have more cows to maintain a carnivore diet, i.e. exacerbate the issue dramatically).
In fact, a lot of the ecological harms will pass once we start producing less of certain chemicals. Take ozone layer as a common example. Ozone holes were spreading rapidly due to use of several interfering chemicals widely used in refrigeration, compression etc. Once they were banned worldwide, ozone layer came back to normal, because these chemicals don't stay there forever. But for the time of their life in the armpsphere they caused massive harm. Same idea.
It is a very well established scientific fact that cows heavily contribute to the greenhouse gas pollution, and that greenhouse equivalent of pretty much any other food that exists is way lower. Like, fifty times lower for some staple plant-based food sources, and 2-4 times lower for other animal-based food. It has been measured many times and is a clear scientific consensus. Claiming cows don't hurt the environment is akin to saying the Earth is flat.