I just don't think you can convince society to give up meat
Significant even if just a portion of your Beef meals are switched to Pork or Chicken. 2-units (by weight) of grain produces 1-unit of chicken, 4-units for 1-unit of pork and 7-units for 1-unit of beef.
Getting meat-eaters to have one vegetarian meal a week is significant too.
Assuming we need to change, then it will take a generation, but small steps are enough to achieve that.
there's a lot of anecdotal reports of vegetarians having health problems after awhile
My anecdotal is that amongst my children's peers (now late teens / early 20s) there are a significant number of kids with dietary problems.Thinking back to when I was a kid I can't think of a single one amongst my peers, except for one kid that didn't like sugar on his Cereal - we thought he was weird!
Theories abound, and I have mine, but no sure idea what the cause actually is ... and given the number of dietitians that my kids friends keep busy I don't reckon science has much idea, yet, either!
I point the blame at the use of herbicides right before harvest. Why would we risk doing that (just to make harvest a bit easier)? It may be more common in wetter areas where bringing forward the harvest by 2 weeks, by killing the grain before it is naturally ripe, and killing any weeds too which no doubt also helps with harvest, but I live in the driest part of the UK and harvest here is never a problem, even in a wet year, but even so the farmers here often use herbicides right before harvest. I've lived through a number of cause-and-effect situations where chemical side effects have been seriously underestimated - starting with Thalidomide which my Mother narrowly escaped taking in the 50's when she was pregnant with me.
I happen to think that Glyphosate is incredibly safe, to humans, as the chemical pathways it effects in plants don't exist in us. But I am very unsure about my gut fauna, and research on that seems to be mighty thin on the ground; the bugs in my gut mostly originated from soil, and there appears to be science that soil-bugs are impacted after fields are treated with Glyphosate, so maybe my gut-bugs too.
Or it might be the rise in Petrochemicals in our everyday life. Stuff leaching out of plastics into the diet. Every darn thing is wrapped in plastic ... never was when I was a kid, and I don't think much of it needs to be.
Seems to me that Vegetarians are bound to ingest more crop-treatment-chemicals than meat eaters (although the meat eaters will be getting antibiotics from animal treatments etc.), so maybe the extra intake of Pulses etc.that vegetarians eat, treated with herbicides "to help harvest", are the problem ...
So in your "wet finger in the air" check on Vegetarians that you know I would recommend separating out the ones who eat a 100% certified-organic diet from those who don't, and see if your two camps show any significant difference.
My kids have had some dietary ailments, not as severe or debilitating as their peers, and they are not vegetarians (although in common with their generation they are environment-aware, so definitely favour some all-vegetarians meals during the week). 4 months ago we switched to 100% certified-organic food in the house (and doing that made me painfully aware of just how many products you then need to be careful of). Too early to say of course ... and when eating out / at friends' houses we have no control over that food, but if we get their Organic intake to over 90% it will be interesting to see if my kids notice any change.