There's a lot more to not-plants than a steak, though. Fish and milk and eggs and cheese and shellfish and who knows. That is a huge variety of food, of flavors, of sources and methods for farming, giving it all up is arguably as-healthy, not healthier, and certainly much, much more boring. Also, harder to sustain, it's easy to find food with meat in it, plenty of places have few vegan/vegetarian options.
It's true it can sometimes be hard to find vegan options, depending on where you live, but the situation has improved massively in the last few years. The number of vegans in the UK has almost quadrupled in the last 10 years, for example, with a corresponding rise in products, restaurants etc:
http://www.telegraph.co.uk/food-and-drink/news/number-of-veg...
For a certain subset of the population, getting the appropriate amount of protein and other nutrients would be nigh impossible since those foods could make them incredibly ill or kill them.
People also mistakenly put way too much emphasis on protein and not nearly enough on other nutrients. The average American, for example, gets way more protein than they need and not nearly enough dietary fiber.
Regarding the topic of too much protein, nutrition doesn't quite work that way. You can't go tick-tock on your diet just because you have too much protein today; you will need protein until the day you die. You also need suger, fat, salt, and yes, fiber.
It's also quite wrong to say everyone could be on a vegetarian / vegan diet because you well know that it's quite dangerous if it isn't followed correctly, and lo and behold, the average American is overweight.
We can sort of blame the new trend diets for the lack of fiber.
Which diet is dangerous again?
Unless you're eating a really weird diet it's almost impossible to get enough calories but not enough protein. Eating a healthy vegan diet is easy and easier now than it's ever been with the wealth of information out there.
> Oral allergy syndrome (OAS) is an allergic reaction in the mouth following eating food.[1] It is a type of food allergy classified by a cluster of allergic reactions in the mouth in response to eating certain (usually fresh) fruits, nuts, and vegetables that typically develops in adults with hay fever.[2]
> OAS is perhaps the most common food-related allergy in adults.
> It is usually limited to ingestion of only uncooked fruits or vegetables
I'm actually susceptible to this to some degree during hayfever season too. Unfortunately I can't eat some fresh, raw fruit during that time without getting a very itchy mouth. It's hardly a dealbreaker for a plant-based diet though.