“Now, if Chazal’s value system was such that human need takes priority over tzaar baalei chaim, and someone comes along and says no, we gave no moral right to benefit from something which causes tzaar to an animal, I think he is a kofer b’divrei Chazal.
Yes, Chazal believed in being compassionate to animals; in fact, tzaar baalei chaim is d’Oraisa according to many. It has its parameters and limitations, though, and one who thinks he has a moral standard higher than Chazal, I think, is an apikores. Some forms of vegetarianism do claim a higher moral standard than Chazal, and that’s much worse than being stupid.”
I was going to object by quoting the Rema but then I saw that you already ????? ??????? a few posts later by quoting the very Rema that I was going to quote. (By the way it’s 5:14 not 5:19) However, you wrote “in a case of extreme pain, the Minhag is to keep the Chumrah to abstain” which I don’t think is what the Rema says. He says that anything is muttar if it’s necessary for some purpose and therefore it is muttar to pluck feathers from live geese. I’m not sure that that falls under the category of extreme pain. Then he says that even though it’s muttar the olam is noheg not to do it because it’s achzorius. If anything this is a proof that even though the Torah allows you to do something, it doesn’t take it out of the geder of achzorius (it just means that the Torah permitted you to do achzorius) and therefore to whatever extent possible/reasonable you should avoid doing such things. This would therefore seem to support the idea of being a vegetarian because “killing/causing pain to animals is wrong” because the fact that the Torah allows you to do it doesn’t mitigate the achzorius aspect of it. Certainly it would not be apikorsus to hold oneself to such a standard. I do agree though that there is a difference between what I have just mentioned and someone who says that it is categorically wrong for anyone to eat animals. Because that the Torah permits.