Hmm, I get your point - I think you’re raising a compelling case.
I think, for me it comes down to the belief that only very, very few dogs are so aggressive and dangerous that no intervention will be able to change that. I (with great reluctance :) )agree that if a dog will never be able to get adopted, it is responsible to think if it would be more humane to euthasize him. But there are also far, far too many cases where dogs are killed because there just isn’t enough money or interest in them to give them special treatment and care so that they can e.g. trust humans again and not see them as danger.
I also agree, however, that it would probably be a good idea to implement limiting measures to the amount of dogs out there, so that the problem isn’t growing in scope - e.g. those you proposed. In the end though that can’t be the solution to the moral question “is it okay for us to kill dogs with whom we haven’t tried all in our power”, it can just be a supporting factor so that we can avoid making these decisions as much as possible.