Well we would not literally die out just a lot people would die. No one know how many people could survive but clearly far less than there are now. Veganism isn't only about not eating some snuff. It's about abstaining from the use of all animal products for all possible use cases. People who call them self vegans obviously all have their own definition because it's simply impossible to not use animal products they may stop at drinking milk or using leather for clothing while other wouldn't even sit on leather. But all of them completely ignore that literally everything around us is somehow an animal product or animals where used in the production. Fossil energy for example that is partially an animal product. It's also made from plants ofc but since you cant separate it you would not be able to use it. By that alone, so by removing fossil fuel from humanity we would already kill a really large portion of humans.
You could spin that even further and realizes that humans are animals too (yes we are ;D ).
The whole concept of veganism is a religion-like nonsense idea that works for everyone who wants it to work because everyone defined their rules how they see fit. But if you look at it from scientific point of view its just people fooling them-self to feel better/morally superior pretty much like a religion.
Yes modern world can synthesize B12 but a vegan wold could defensively not. Its just very very ignorant if someone thinks that there is no use of animal products in the production line of synthesized B12.