My take?
People put way too much stock into diet and how it may (or may not) influence their running and overall health. They change one little thing in their diet and then if something happens in their running the next week (or their subjective feelings of energy levels) they will often attribute it to that one change in their diet. Often times you hear people obsess over things like "brain fog" or the subtle condition of their skin.
In reality that are many variables at play (some mental/placebo effects). Life stresses, sleep patterns, other lifestyle attributes that are probably going to effect their running and energy levels even more. No doubt diet is important, but Running Training and genetics is probably going to be the most important for your running speed. I used to hang out with 2:10 marathoner Brian Sell and he'd eat McDonalds all the time....he also ran 160 miles a week.
I grew up as a ovo-lacto vegetarian and I thought vegans were nuts (pun intended).
Now that I am vegan I don't think that anymore, but I'm also not going to pretend that humans didn't evolve as omnivores. I still think in a lot of cultures meat is totally overrated (and dairy and eggs for that matter).There is a huge financial stack to push these things (government funded and big business).
At this day and age though (at least in most first world countries), I believe it is quite possible to eat vegan (plant based) and get everything you need...even as an endurance athlete. I wouldn't say that was the case 50 years ago...and it is not the case in a lot of 3rd world countries.
People always bring up "bioavailability and quality of protein" but generally that is pretty BS. It's basic science, biochem and nutritional science. If the body can't tell the difference where it's protein sources are coming from (be it a grain like quinoa that has all essential amino acids).... does it really matter?
There are certain things (like B12, iron, Vitamin D, DHA/EPA fatty acids etc), that I think all athletes (regardless of diet!) should check into and maybe consider even supplementing with. I think it's very hard as a high level endurance athlete (regardless of diet type!) to stay on top of some of these things while training super high mileage. Like I always say, some of the most iron deficient runners (and injury prone for that matter) I've known have been guys that eat red meat 3 times a week. But again diet isn't the only thing influencing this.
I don't believe humans "need" to eat animal meat/animal products. Obviously most people do and they enjoy it and enjoy the taste. Most people also just don't eat a very balanced diet in general though and that's why countries like the US have such bad obesity rates, rates of heart disease, and cancer.