I've been vegan for around 6 months now, and haven't dropped dead yet. If anything, I feel the same as I did before - on a vegetarian diet - and before that - on the standard Western diet.
As long as you eat a balanced diet with enough protein, fat, and carbs humans can thrive on a vegan diet just fine.
How many super overweight people do you know who eat all vegetables and whole grains?
There are always outliers but the cliche western 'bad diet' is too much meat, fat, processed carbs and dairy.
I'm a reluctant part-time vegetarian - pizza is my spirit animal. But I cannot ignore the cognitive dissonance I experience with a lot of the 'bargaining' style diets being pushed. My animal brain really wants to believe there is a healthy diet that lets me pile on the bacon and chorizo and still lose weight but the evidence just doesn't support it.
Too much refined sugars + animal products = fat population.
The only nutrients "missing" are B12 and D, which are easily supplemented and/or found in fortified foods (cereals, plant milks etc). And the funniest bit - they only exist in meat because they are supplemented into cow feed. That's right - you're already supplementing D and B12.