Culture can be changed - we changed the attitude toward and prevalence of cigarette smoking in 2-3 generations, we could change other practices as well. Success doesn't need to mean 100% adoption - material changes in attitude toward meat consumption (luxury/celebration vs everyday) would make a material difference in the overall impact. And yes, some/many ranchers, butchers, etc would need to turn to some other industry for income. Solar farms should do pretty well on ranchland. Full disclosure - I'm NOT a vegetarian and love a nice steak. My doctor has recommended I incorporate more plant protein in my diet. But I'm not really trying to convince anyone that they should do the same. I'm mostly reacting to the opinion that it's impossible/silly. My 2 pack a day smoker dad thought the same thing back in the 1960s. By the time he was in his 70s, he was a firm non-smoker. Things can change.

Edit: I'm also not trying to equate eating meat with smoking - one is obviously more harmful to society than the other. I'm just thinking about the cultural change that happened. Culture is malleable. Laws aren't a great way to affect culture, but social pressure is really effective.

Culture can be changed indeed, as can be seen in religion where branches has happened and whole demographics has gone and switched to different religions.

The reason for the decrease in smoking is still being researched and debate, but the main arguments that I have seen is the main role of smoking as social stress relief and as such it has been replaced by other addictions like opium use (mobile phone use, social media, and games has also been argued).

If we consider food culture to be malleable, we definitely should move to aquaculture as a main source of nutrients. Farms on land compete with forests and human activity, and land based farms uses artificial fertilizers (derived from natural gas) and massive amount of fresh water. Shellfish, invertebrates, and seaweed can provide both protein and calories, and has the lowest environmental impact of anything we eat today and can even have a positive environmental impact if farmed correctly. Changing farming from doing harm to doing good would have massive impact, and all it takes is a cultural change.

I think the difference is that smoking is also horrible for the smoker, and many want to quit, but can’t very easily. People don’t want to stop eating meat, driving their SUVs, buying cheap shit on Amazon, and flying to cool vacations. Stopping doing those things doesn’t benefit them unless everyone else stops too, and unlike smoking, we’ve made little progress in the last few decades (other than the progress unlocked by technology that means it’s immediately beneficial to the one making the change, like solar), so I can’t blame anyone for thinking that giving up meat to save the planet is stupid since their entire lifetime of lived experience tells them that the vast majority of their fellow humans are not going to do the same.