Why Vegetarianism is morally right
Your argument for ecosystems is exactly why vegetarianism is morally right.
Factory livestock farms aren't just cruel to animals when they're slaughtered, but for their entire lives. Imagine if your entire life was confined to small bedroom with 30 other people stuffed in with you.
You'd go your entire life without ever spreading your arms full length, and be force fed food that only fattens you with nearly zero nutrition plus massive amounts of
Antibiotics to keep diseases from killing you all off prematurely. Because of your added hormones and force fed diet your intestines may even explode with your gut hanging on the ground for the rest of your short life.
But without factory farms, there wouldn't be enough cattle available to feed the worlds 6 billion people if raised in a natural cruelty free grazing lifestyle. Only if a much larger portion of the human population were vegetarian would this be possible.
Then again, if inhumane livestock techniques were eliminated, the cost of meat would increase to the point that a more vegetarian diet would be the only option for much of the worlds population. IMO, a win/win.