Humans on the other hand are perfectly capable of living on plants
Plowing kill animals too.
It certainly doesn't repeatedly breed them purely for the purpose of killing them later, killing billions and billions of animals over many years, no. Do you think the population of cows was so high before humans started breeding them en masse, cutting down huge parts of the Amazon rainforest just for pasture?
If the natural food-chain was efficient, no life would exist whatsoever.
Plant life would still exist, because plants take energy from the Sun, and the Sun is not alive. I see your point though.
In terms of efficiency we'd be better off with nothing.
Yes, but we live in the real world, so we should be as efficient and rational as possible while still being alive. I understand that people can say "I just like meat", but the damage that animal agriculture causes far outweighs any petty pleasure that people can draw from it.
The last point really depends on the scope at which you consider the consequences. Animal agriculture has contributed to increased CO2 emissions and thus global warming. Simultaneously it has caused a lot of suffering for animals. As a result, there is a massive amount of research and development going on that seeks to alleviate these symptoms, which will ultimately make the world a better place once ripe. People will be able to eat meat without having to put animals through shit. Since the meat will be lab-created, it will also be possible to make it healthier and tastier at the same time.
While I do disagree with the animal abuse that goes into "creating" most of the meat on earth, I do think that it helped a lot of people survive. It's easier to transport large amounts of meats than to transport orders of magnitude more vegetables and fruits. Transports are still extremely expensive after all, and also contribute largely to the CO2 emissions.
Furthermore, traditional agriculture benefits the environment, rather than harming it. This is due to the animals essentially creating fertile soil in which plant life, critters and rodents and whatnot can thrive. This also creates a sustainable environment for birds, reptiles and other animals.
Which is also why I not only don't see any moral issues with eating meat that was properly farm raised, but actually consider it as beneficial.
The non-plus ultra will be raising a few animals that provide all these benefits, while providing the cells needed to raise meat in labs (without hurting the animals!). We are definitely getting there, and it's just a matter of public awareness and funding as to when we will reach that point.