- Land use (over-grazing, deforestation for pastures and feed crops, most of crops grown are for farmed animals, etc.)
- Water use
- Inefficient feed -> food conversion
- Nutrient runoff leading to eutrophication
- Increasing risk of zoonotic diseases and antibiotic resistance
- Bioaccumulation (of PFAS, pesticides, heavy metals, microplastics, etc.)
1. Pandemic evolution
2. Antibiotic resistance
A distant third is GHGs / climate change, and then the other issues that matter only if we stay alive.
In many cases animals graze on marginal land that can’t be used to grow human food directly. In these cases the alternative ways to get the same protein production could imply cutting down land in other countries to grow protein rich crops for humans.
Water use doesn’t have to be high. In the US it’s a product of industrialised animal agriculture, bad subsidies and free water rights that farmers are forced to use unless they want to use those rights.
Animals can convert food that humans can’t eat into food that we can eat. In some cases they eat the part of the plant that we don’t.
EU has prohibited all routine use of antibiotics in farming. Other countries should follow.
And there are upsides of animal agriculture as well. They are often a critical to do regenerative agriculture. In the best case they eat grass from land that we can’t grow stuff on, eat parts of the plants that we can’t eat ourselves, and give us high quality fertiliser that greatly improves soil quality. There’s a reason farm animals have been with us for millennia.
That said we should absolutely eliminate all heavily industrialised animal agriculture, which means we have to reduce meat consumption. It would be interesting to know how much meat we could eat if all meat production was sustainable.
Just to make the implicit explicit: the current reality of meat production is feeding livestock with protein rich crops grown on cut down land in other countries. An alternative’s model scalability and economic viability has not been shown yet.
So the hope of such a thing existing in the future should probably not influence how you choose to nourished yourself today
As you implied, continuing current levels of meat consumption for 8 billion humans, even with regenerative techniques, will still result in substantial increases in land/water use and other negative impacts. We should be sceptical of regenerative grazing claims as well, as they are often pushed by the industry, without sufficient evidence.
I'm hesitant to say just because we've been farming animals for thousands of years is good justification for continuing it (at least at this scale on non-marginal lands). After all, it did lead directly to the problems we face today, as people clung on to traditional methods without thinking even as they migrated and new technologies/knowledge became available. For example, European colonists brought hard-hoofed cows, sheep, goats, horses, and deer to Australia, despite the country's native fauna all being soft-footed, leading to soil compaction and many resultant ecological issues. Now almost 50% of Australia's land surface area is used for red meat farming, and it is the leading cause of deforestation in many parts of the country.
On plant parts that humans can't eat, perhaps it's fine to let wild animals eat them, further improving biodiversity (after all we need a healthy biosphere of many species providing ecosystem services for long-term human survival), or we can compost or process into other products. I'd be looking into stock-free organic farming and precision fermentation as new promising approaches as well.