“I am much more inclined to believe that they decided on vegetarianism the same way Mathew Scully did, and then looked for support from Torah authorities.”
I don’t know what Scully’s motivation was. But if the motivation is just a general desire to be more compassionate (as opposed to an anti-Torah view that korbanos are inherently wrong or whatever), I don’t think we should condemn that, since the Gemara repeatedly says we should emulate’s Hashem’s traits, especially compassion. On the contrary, it should be praiseworthy to go beyond the letter of the law to be more compassionate (just as it’s a kiddush Hashem to return a lost object to a non-Jew even when it’s not halachically required).