Diet has historically been closely tied to religion, cultures, family traditions and even "favorite foods" as a child.
Hence why people are so passionate when talking about it.
I was born and raised vegetarian and thought the vegan diet was "extreme." I've been about 99% vegan for the last 3-4 years.
Sometimes I try fondue in France and eat a nice baked good that has eggs in it. Sometimes I probably accidentally eat some bugs that have been killed in my salad/fruit. I eat honey. I'm not going to be a stickler about it. The healthy diet ideal is "whole foods plant based" because then you are talking about whole grains like rice/oats as well as the focus on veggies, fruits and legumes/nuts. Obviously one is not going to be healthy eating vegan donuts and vegan Ben and Jerry's ice cream all the time. We've found saying "plant based diet" is less offensive to some than saying "vegan"...which tends to trigger people.
The whole Environmental angle is a big one. Simple math and science shows that being on a plant-based diet is going to be far more efficient (generally) and reduce greenhouse gas emissions. With projected overpopulation growth there is no way most people in the world could even approach to eat as much beef, dairy, seafood, pork as the average American does. Not even in the US can everyone eat "' healthy' grass fed beef"....it is simply not scalable or sustainable. Without government subsidies the true price of meat/dairy would increase drastically. And of course most grain in the US is used to feed livestock....not people.
Now I personally pollute a ton....I've gone on half a dozen international plane flights this year, I drive a gasoline powered SUV and I take 10-min hot water showers a lot of the times. There are much better Environmentalists out there that i greatly respect....but diet is a huge component of that. Usually more so than even all the transportation choices one makes.
With vegans you have a fairly outspoken minority. And thanks to social media that minority is growing into a pretty substantial market....there is a reflection of that in products you see at the store and labels on restaurant menus. Being vegan (or more importantly "plant-based") will continue to grow worldwide as people recognize the health benefits and environmental benefits heading into the future. At this day in age there is no reason a person can't thrive on a vegan diet..
As for B12? Iron? Yeah, I think all Runners (regardless of diet) should monitor their B12, Iron and Vitamin D levels (to name a few). Some of the most iron deficient runners that I've ever trained with were guys that ate red meat 3 times a week.