I’m perplexed by the appeal to opportunity cost because it seems unlikely that in the world where OP decided not to care about the ants, he would have used the time to make money to donate to effective charities instead. More broadly, I worry that it’s easy to appeal to opportunity cost without demanding sufficient rigor of the alternative. For instance, people often say that avoiding causing animal harm through poultry and egg consumption is inefficient without seriously investigating the counterfactual. At least some of these people are wrong about how much suffering their decisions could avert and how little the changes would cost them.
It seems to me much safer to lay the burden of proof on the moral indulgence—at very least, the burden of proof shouldn’t always rest on the demands of conscience.
I don’t understand the distinction here between rights and “not being treated any old way.” Is the argument that the imperative not to make animals suffer is lesser/more contingent because of their lack of agency? What is the practical takeaway of “animals don’t have rights”?