Smallpox harmed the people it infected; hypothetical useless unloved resource-draining methane-farting future cows (HUURDMFFC) harm the people who could benefit from the resources they divert and who want to live in a world with less methane. This seems like a quantitative difference to me, not a qualitative one.

I don't know about you, but I like snakes and I would be sad if boa constrictors went extinct.