cows are a natural part of the ecosystem

Sure large herbivores were and still are part of many ecosystems.

But around where I live the majority of the grass for the grass-fed cows doesn't come from anything remotely resembling a rich ecosystem. The grass is literally 'grass': maybe one or 2 types of grass, similar amount of herbs, funghi. Hardly any insects except for flies attracted to manure. These used to be ecosystems with > 20 species of grassses and herbs per square meter.

And these are even relatively small farms; trying to upscale it beyond that to make it possible for millions of humans to eat meat multiple times a week, it won't get any better. If you're putting large amounts of cows in a much much smaller habitat then what they'd naturally use, then it's not the same ecosystem anymore.

Its perfectly possible for grass plus grazing animals to be carbon sink, and a provide a rich ecosystem.

tldr; yes, but only if you want to feed a couple of people from it.