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.
“The Enemy wants to bring the man to a state of mind in which he could design the best cathedral in the world, and know it to be the best, and rejoice in the fact, without being any more (or less) or otherwise glad at having done it than he would be if it had been done by another. The Enemy wants him, in the end, to be so free from any bias in his own favour that he can rejoice in his own talents as frankly and gratefully as in his neighbour’s talents—or in a sunrise, an elephant, or a waterfall.” CS Lewis, The Screwtape Letters.