I don't really eat meat myself but I don't believe you, sorry. What about "if you ate meat you could increase your brain capacity by 500 cc and live a longer, healthier life". Would that make you go back? I bet it would. Even if it wouldn't, it would probably make the majority of people go back for sure.
You cannot seriously make an argument that we as humans should potentially accept any diminished quality of life in regards to our mental capacity and health to "reduce the amount of atmospheric carbon".
Again, as I said, I don't really eat meat and I'm all for saving the environment but arguments like that are the reason why it's so hard to convince "ordinary people" of the importance of sustainability and long-term thinking. The only alternative is only presented with these radical, self-destructive requirements.
Why not pour the all the efforts of vegan evangelism into sustainable lab-grown meat? If it works, all problems solved, everybody can eat as much meat as they want (or not), the environment is saved and a lot of (potentially increased) brain capacity freed for other human improvement.