...replaced by monocultures...

Guess, what? We have those monocultures today. That's how we feed cattle. But if everything went vegan we would be able to cut that land usage down to 75%.

Veganism is about killing as little as possible, it's not about being 100% perfect. We would need those farms to feed people, and of course animals would die in the process. But it's a smaller harm.

No doctor will tell you you can live well on a vegan diet without any supplements.

Says who? Here's a quote for you (source):

It is the position of the American Dietetic Association that appropriately planned vegetarian diets, including total vegetarian or vegan diets, are healthful, nutritionally adequate, and may provide health benefits in the prevention and treatment of certain diseases. Well-planned vegetarian diets are appropriate for individuals during all stages of the life cycle, including pregnancy, lactation, infancy, childhood, and adolescence, and for athletes.

You're right, we would need supplements, like B12. Today those supplements are given to cattle, because they can't get it when they don't graze. That's were you get your B12: from supplements given to animals. Maybe we could skip the middleman?