I don't think it's possible to make an argument that eating meat is somehow the more morally sound option than not eating it. Though if you're not convinced by the moral argument against it then that's fine.
animals have to eat living things to survive, and whether the things they eat have a brain or not is pretty much incidental to your biology. humans are equipped to eat meat naturally
The fact that other animals do it or we have evolved to be able to do it doesn't seem like good justifications to me. Animals do plenty of things we would find immoral. We as a species have evolved to be able to reason about what we should be doing and as such we can choose to go against our instincts.
MF said that we can't know for sure how other living things experience life, which is definitely true. However we can assume that most animals are sentient and are capable of experiencing fear, even if they aren't able to comprehend death. If it's possible for me to minimise the amount of suffering I know my choices will cause, I will do that. If some study in the future finds out that all plants are conscious in the same way as we are, then I will re-examine my position.
Greenhouse emissions isn't the only effect that animal agriculture has on the environment. One is sustainability. The world's population is increasing and we need to feed them all. While a lot of famine in the world is due to poor logistics or (even more sadly) poor profit incentive, the fact is that raising animals for meat is a very inefficient way to get energy into humans. For cows, you don't only need the land for the cattle themselves but also the land and resources needed to grow the grain which feeds the cows. If we stopped farming as many cows, that's plant-agricultural land we've freed up to feed humans. And it would be able to feed more humans than the animals they would have fed otherwise.
And another problem is antibiotic resistance. This practice is banned in the EU, but in China and I think the US, it's cheaper to pump animal feed full of antibiotics instead of actually taking care of your animals. This increases the risk of antibiotic resistant superbugs emerging.
Despite my last couple of paragraphs though, I actually don't like focusing too much on the environmental arguments. If it gets more people off meat then that's good, but it doesn't sit right with me because I don't want to imply that eating meat would be okay if it weren't damaging the environment. The animal welfare argument is so important to me that I would not eat meat even if it were somehow better for the environment.
I live in the UK where there are laws that say that food that is suitable for vegetarians should be labelled as such and every restaurant has to provide at least one meat-free option. It isn't much but this makes it much easier than it is in other countries. People who are poor or don't have much time on their hands might not be able to go to the effort of reading ingredients of everything they buy or might be forced to eat a lot of fast food. So while going meat-free is generally cheaper if you can cook for yourself, the people who don't have that luxury can be forced to have a meat-heavy diet if things like those laws I mentioned aren't available to them.
And I think there's been a snowballing effect recently. Veganuary has been getting more and more mainstream every year I've noticed. This year, I couldn't walk down the few streets near where I work without seeing several signs advertising vegan options. The more vegan options there are, the easier it is for people to make the change, which in turn raises the demand for vegan options. So I'm happy with the direction things seem to be taking, at least in my neck of the woods. I know that London isn't exactly representative of the country's population as a whole.
What I think will really change things though is when either lab-grown meat or plant protein based substitutes which try to emulate meat in both taste and nutritional value become cheaper or more profitable than actual meat. Once it's cheaper and it's more convenient for everyone to just get the meat-free option, I expect that "suddenly" society will realise how inhumane the whole industry is.
Anyway, myself, I just want to minimise the amount of harm and suffering in the world. And it's much better and easier to convince 10 people to reduce their meat intake than 1 person to go completely vegan imo, so that's why I'm not going to go around telling people to change their lifestyle completely.
