I agree that this argument does boil down to, partly, your views surrounding the concept of anthropocentrism (google it).
If someone holds 'green' principles to heart then they will be more likely to be vegetarian/vegan.
People can do as they please.
I do find it odd when some vegetarians eat fish (some even eat chicken on occasion) but whatever.
Personally, we create these animals to eat. I enjoy most meats, i feel it is providing me with a good diet, i do believe we over humanise animals and as long as we don't cause unnecessary harm when slaughtering or treating them then i see no issue.
I bottle fed 5 lambs when i was a littke boy. Foot and mouth killed them. 'Nature', in other words' had them all burnt on the field in front of my house.
I don't think nature had a conscience.
People should stick to their principles etc but should not try and change the world. People can do what they want (excluding breaking 'common' laws)
Personally, i think it will be harder to meet your nutritional needs of you exclude meat.
Staying pretty skinny, eating very cleanly and doing some light exercise will mean you will, quite possibly, live a pretty long life with reduced (not eradicated) risks of major killers. You could still get run over, or shot or get cancer etc.
However, cba with all that. I don't frankly care if i don't make 90. All about memories and enjoyment. Too many variables to care.