For as long as there is demand for beef for consumption, then there will be cattle reared for that purpose.

Yes, but the comment was If (ever, I may add) the demand would drop or disappear, because people turning veggies, it will happen gradually so only gradual breeding reductions will keep the balance and all the meat cows would be eaten, so none would be left without irs purpose.

But my guess: I dont see this as even being chance to ever happen.
If some day all people turn veggie, it would way more likely be because something bad happen and all the farm animals would be gone. But even that is very unlikely... Because even at "doomsday", the surviving would bring some animals for food into the shelters too. Probably some more universal breed than the todays most common in mass farming dedicated meat- or milk-only, but there still will be milk and then beef as a food.

Breeding animals for food is extremely wasteful use of resources (when the same land area, water, time, work investment etc. could be used to grow plants that can be directly consumed by humans). It is probably not sustainable already nowadays, let alone in a post apocalyptic world

In the future, Mars, the moon or a spaceship animal products for food will be the least option, because of the inneficienty of it, as you say. So an hypothetical moment where everyone is veggie would be caused by a surviving need (in another planet, spaceship or an earth low on resources, water or oxigen) rather than a religious, trend or culture reason.

Meat production will be way too wasteful for the near future, because almost all the water and energy put in the animals is wasted as methane for example, and you can't just stick a bag to a cow to collect the methane, or put them in a chamber.

If you are breeding animals each breed just for one purpose (what is the case with todays industrial farming), then maybe.
But that is an anomaly of last few decades.
More "normal" was to use e.g. the sheeps for dairy, wool and only when they get older, for meat and the leather. Then you get multiple products (not only food) while still feeding just one single animal.
The thing is: Animal farming is 1000's years old. Coming from the time any resource was hard to get, yet it proved to be the viable method to get food and supplies. If it were that inefficient, it wouldn't be a thing in the first place.
You should not forget not all vegetables are processable by humans directly. And those which are, do not grow that easily. Taking the animals means you become able to ultimately get food and other material supplies from vegetation which tend to grow easily but would be useless for humans otherwise.
I dont think the "postapocalyptic" environment would change anything from those equations...

Over the last few decades human population on Earth grew to a point that broke quite a few balances of use vs. availability of resources

The traditional farming of animals maybe is not "inefficient" compared to industrial farming in raw resources, but it is "inefficient" economically vs. industrial farming, which drives it out of existence

Also, humans nowaday overuse resources compared to the past even when normalized to the population size. The classic example is production and trashing of largely unnecessary and/or short lived technoogical devices. In the context of this discussion there is also the over consumption of meat, compared to time when it was not produced as a separate product

In the early days, resources were difficult to get, but with some effort, trading, exploring you can get them. Unlike the early days, in the future resources will start to extinct, no matter how hard you peek at every meter of earth you'll not find much of them as before anymore.

Of course they will not fit a couple of cows in another planet or a spaceship, as the cows needs to grow and reproduce, and that take several months or years, so at the moment when the cows are ready, the crewmate already starved, and you need to control the waste. Maybe fitting two chickens in a spaceship would help, for get eggs.

Breeding animals for food is extremely wasteful use of resources (when the same land area, water, time, work investment etc. could be used to grow plants that can be directly consumed by humans). It is probably not sustainable already nowadays, let alone in a post apocalyptic world

That's all very well, but if you get a severe weather event or something else that destroys the food crop, plagues of locusts and the Potato Famine spring to mind and you are in serious trouble.

A balance between the raising of animals for food and other resources and crops is what's needed. That is the system we have today, and it's evolved over thousands of years.

There is no such balance as if you have no crops, you have nothing to feed the animals with, then you have no animals either. Every emergency backup solution you might come up with would have to be as efficeint as possible, which flat out rules out any animal involvement

There is no such balance as if you have no crops, you have nothing to feed the animals with, then you have no animals either. Every emergency backup solution you mright come up with would have to be as efficeint as possible, which flat out rules out any animal involvement

You have missed one big point:
People are omnivores, so tyeir digestive system is set to a kind of compromise between efficient processing of herbal food vs not letting the meat food to spoil. As a consequence, people are able to process only some plants or plant parts, or need acomplex industrial infrastructure to process it so it becomes an useable food. So we have to rely to few specialized crops, usually relying to the similar complex industry to get some usable yield.
If some disaster strikes, the fragile complex industry would be gone, when half of all the plants generally would be gone, you have big problem.

The herbivores have their digestive system so they are able to process pretty everything that grows around, so may swith to whateve plant just happened to survive in quantity, so no big deal even when half of the plants are gone. But when farming these animals for human food, you get maybe less efficient, buy way more reliable way to get human food. Your primary food chain source becomes virtually whatever happen to grow there, you just use the farm animals as a means to process that into something usable as human food. And a herd of sheeps or cows is way likely survive in an extent to be able to act as the food processing for you than a complex machinery with all its supply chains of a food processing fctory you would need otherwise.

We get to choose which crops we keep "backed up" in safe storage for a disaster case. The backup can be done in a number of levels :

 - Seed vault, like the Slavbard global seed vault

 - Hydrophnics grow in a shelter, so there will be a quantity immediately ready for use when the disaster strikes

 - Normal (unprotected) grow in fields and greenhouses, but distributed in many different areas, both on country and global levels. (Assuming the area of destruction of a disaster is delimited by geographic limits, and not by affected species of crops)

 - Normal (unprotected) grow of wider assortment of different crops. (For the case of some biological disaster that affects specific crops only). If in normal times there is not sufficient demand for those crops by humans, then use them to feed the animals

In all of them, we choose what to keep "backed up", and so we can choose to buckup mostly the types of crops that can be used directly for human consumption. We can also focus on crops that only need "low tech" processing before they are ready for eating

We also have the evidence that :

 - Vegan societies had existed for millenia before any industrialization. (Maybe not in the modern sense of vegan where you dont eat any animal product at all by desire, but just having inconsistent and far inbetween animal product availability, so for all practical matters still vegan)

 - Within the group of vegans nowadays there is also a sub group of "natural only" vegans to a set of different definitions for "natural only", one of which is "unprocessed only". It shows that a set of crops that can fit all the reqirements (human consumption, no high tech processing) does exist



We do need to backup also the animals and the crops that are good for animals only, so they dont go extinct in case of a disaster. But i mean this does not have to be in quantities intended for actual mass consumption, if crops alone make a more efficient option (so can last for longer in the time when a disaster strikes, before it clears and recovery work can be started)