Most people have consumed animal proteins the catch is they weren't eating a lot meat regularly. One of my ancestors was upper middle class when he migrated to the USA and founded a small city. His journal talks about his meals and as a wealthy person with a dairy farm he still mostly had stew and rarely ate steak. He wasn't eating animals all the time and that was 1840-80s. We need to go back to a time when eating a whole chicken among a family of four in a single meal almost never happened IF we continue farming animals which as you noted is a climate change nightmare.

1. Humans have also been eating meat for millennia. We're omnivores, and have always been opportunistic about what we eat
2. Individuals cutting down their personal meat consumption won't stop factory farming, but ending subsidies that make that practice highly profitable might