And that's the hard part as a dog owner who seriously loves dogs.
While we see them as so much more than just another animal, and we want to humanize them and treat their lives as if they were another human.. they are still just another animal.
I say the same thing about humans: euthanize the weak ones.

But, I am in the minority with that opinion.
But while pet dogs deserve so much, you have to look at it responsibly.
Hell, the owner of a dog that has such a terrible affliction, should look at it positively. While you may never get to see what kind of positive benefit that one specific dog may bring to a home, by giving it the treatment of euthanasia to end needless suffering, you are afforded the opportunity to adopt another rescue animal and prevent a needless loss of life.
It sucks, but it's the same whether the dog is 15 or not even 1. If it has a condition that ensures the dog is always suffering, do what is right and end the suffering peacefully.