So as I said above, there's a certain degree of actual feedback in terms of your body "Wanting" foods that contain proper nutrients, and while with a combination of multiple types of beans, supplements, and other foods you can approximate some of the proteins you'd be missing out on by not eating meat, it's both far more complicated (and until recently in my life, expensive), and as my initial statement about a lack of "True Understanding"(tm) of necessary nutrition tried to state, has potential long term effects. (and I mean biologically long term, multiple generations. If there are meaningful long term studies on what various exclusionary diets do across generation I'd like to see them, but my understanding was that our data was currently very insufficient.) So I'd respond, there are some constraints that don't say I _have_ to eat meat, but certainly shift the cost/benefit analysis more in its favor.
With the exception of gym rats or those with a job that requires strenuous physical labor, the body doesn't require that much protein.
I've been a vegetarian for 15 years or so, and would question your statement that finding non-meat protein alternatives is complicated.
It sounds like you're trying to make an argument that not eating meat could have long-term health hazards, in which case I'd argue that it's easier to prove that meat is bad for you than it is to prove not-eating it is.
I don't think I'd draw the line as high as "gym rats." I was brought up in a family with a vegan mother, and at home generally had to conform to that same diet. During university, I had a majorly carb based (very unhealthy as well) diet. Across all three, when I finally had the money to buy good quality meat on a regular basis (3-4 times a week), I not only felt FAR better, but my rather light workout regimen (2 sessions a day, 3 reps of 30 set's of a variety of free weight exercises), became FAR more effective. I put on ~10-20 pounds within a few short months, and found myself far more capable in terms of both performing and recovering from the rather simple physical tasks I do as a sysadmin (staying on my feet most of the day, racking rather heavy servers)
Now, this is all anecdotal, I was just addressing that my body responded in a way that suggested that I did "require" the protein.
With regards to the more formal argument, I'd word it more that you put yourself more "at risk" for certain health hazards by cutting ANY core part of the diet out. It's certainly easy to prove meat is "bad" but most of the studies I had seen involved excessive consumption, which defaults back to the general statement of "all things in moderation". Do you have references that show the former (negative effects) without the latter? (ineffective dietary balance).
To summarize, I'd agree if we're only talking in extremes, that it's very easy to show that meat is bad. So I'd qualify all my previous statements with "a well balanced diet"