Overall, I agree with you very much that when people take on responsibility for a living being they should do what they can to make the situation work and I honestly do not like euthanasia in many cases of convenience. That said, I think it was summed up well before by saying it can be the lesser of 2 evils in some cases. When you only keep small fish though, it rarely comes up because it is generally very easy for people to rehome small fish like mollies and tetras. It seems to be more of an issue with bigger, more aggressive types of fish.
But... jet is correct about breeders culling. Also, even if you don't use feeder fish, there is fish meal in fish food. I am not entirely sure how it could be justified to say that feeder fish are more cruel for captive feeding than the process of fishing which gives us the fish used for the meal in fish food. That said, I choose not to keep species who would require live feeders because I don't like it. But, I suspect I am being somewhat hypocritical here. I mean, if every fish ground up for fish meal was humanely euthanized, this would be a different debate. But, they are not. I still don't like feeding live animals to other animals in glass boxes though, personally. JMO.
Two roads diverged in a yellow wood, but thankfully they both led to fish stores.