Have you watched the show Alone? You should check it out. Without modern agricultural systems, humans cannot survive in most places on this planet without consuming other creatures. It is simply not possible to get enough calories. Therefore, if those modern systems shut down, most people would starve without fish, other meat, etc. Is that would you promote? A person can only live as a Vegan because modern agricultural systems make it possible. What happens when, not if, those systems shut down? Are you going to break down as the vegans did on the Alone show and eat meat? Or are you going to starve to death?
Yes, a lot of the developed world relies on agricultural systems because humans made the mistake of choosing to live in environments which are not suited for them, as well as becoming dependent on agriculture instead of also forging for food. By choosing to live in these places you are essentially hoping industrial systems will work to sustain yourself, this is true for many who live in cities. It would be probably be cheaper if people in temperate and cold climates moved to sub-tropical and tropical areas where our original food source is abundant. The masses of people wouldn't do this I bet, so their best bet is to rely on industry hoping it all works out.
Like I said previously, I believe a big reason the masses of people don't become vegetarian is because they come from an environment where eating meat was the only way to survive for thousands of years, so they think it's the natural way of eating and that the rest of world is like this.
This is an uniformed idea and misconception about veganism/vegetarianism. A great part of people in the Indian subcontinent have been vegetarianism for thousands of years without relying on modern agricultural systems. Also, you don't need any form of modern agricultural system to feed all the peoples on earth. Permaculture and the principles of "do-nothing" natural farming could easily work if massively adopted, not to mention fruit trees can be grown in vast amounts spread around everywhere easily. A single avocado tree produces 200-300 fruits per crop after only a couple years of planting and only requires one seed so you could have 300 trees from a single fruit; they are relatively small, calorically dense and imagine how many people could be fed in a couple acre field of avocado? This is just getting started, think about bananas, melons, papayas, cherries, etc. Food is abundant we've been lied to.
This is a parallel and reminds me of when statists say we need the services of military, police, teachers, other bureaucrats, and the system of taxation or else society would be in chaos.
All of what you said only works in certain places on the planet. You said it too. I completely agree that it is possible depending on where you live. A vast majority of people living on this planet right now would be forced to move if they could no longer depend on modern systems and meat. I'm all for people doing that too, but we can't force them to do it.