I don't believe that the environmental argument is quite as clear cut as some people think, and I would love to see a genuinely independent assessment of it.
We would still need agriculture even if the whole world population was vegan. There would be the same constant demand for water and risk of soil damage and fertilisers. Not all land is suitable for growing crops.
There are other environmental factors, for example, most of the almonds that are consumed globally are grown in California. The mechanised harvesting has caused a huge problem with lack of biodiversity, and then you have to add on the food miles on top of that.
Not being funny, but vegetarians and vegans do expel more methane than omnivores. Not nearly as much as cows, but it still has to be taken into account, especially if everyone was vegan.
Vegans don’t have to take supplements
Maybe not, but even the Vegan Society recommends, and even promotes its own dietary supplements, and warns of the potential risks of missing out on some vital minerals and vitamins.