I agree with you that humans in history have had very varied diets. Some humans have had a very long history of lots of meat and dairy. Some have a history of eating grains, pulses, vegetables and other plants. It really depends on what point in time, what civilization and which geographical area we're talking about. Humans are remarkably capable of adapting to a very wide variety of diet when they need to. At any rate, I'd like to show a chart:
https://www.epa.gov/ghgemissions/sources-greenhouse-gas-emis...
And this is just in the USA. Only 9% of emissions comes from agriculture. Even if you cut out all of that, you don't even nearly make a dent in greenhouse emissions. The real challenge is figuring out how we clean up our transportation, energy and industrial infrastructure. The problem is, consumption doesn't necessarily have an impact there.
I think I principally just really disagree with you about consumer activism. I think that the reason why regulation was needed in the first place in order to accomplish many overall wins was because consumer activism is just too weak. Long term, externalities will produce a race to the bottom that is disastrous for society -- individuals don't give things up for moral reasons, they give them up because of incentives. I think we need to close the feedback loop and take the negative externalities of all of these contributors to climate change and integrate them back into our financial, political and social incentives system.
But that requires us to take aim at businesses. Big businesses, of course, would rather us take aim at each other as consumers. If consumers are busy policing each other, they'll get distracted from ever holding the big businesses to task when they're not doing their fair share. And of course, it's scary to hold them to task. They have endless resources, they can bury individuals in court, lobby politicians, spend on PR campaigns that serve as risk mitigation for their regulatory costs. It's much safer to change your behavior as a consumer and moralize other consumers if they don't change their own.
But again, it has limits. Once you get beyond those limits, consumers do not have influence. But, consumers are citizens, and citizens have influence over their country. They can participate in their communities to perform direct action, they can run for office to effect political change, and they can organize to vote for politicians that can make changes that help society run better. I believe in there being more than consumerism to citizenship (not saying you don't).